Monograph No. 55










Chapter I Introduction   1

 1. Circumstances leading to the investigations   1

 2. Organization of the investigating team  3

 3. Publication of this report  6

Chapter II Natural and historic settings  7

 1. Natural topography of the vicinity  7

 2. Sites in the surrounding area  8

Chapter III Investigations at the site   15

 1. Outline of the Excavations  15

 2. Excavation logs   18

Chapter IV Site description  31

 1. General overview  31

 2. Archaeological features  32

Chapter V Artifacts   67

 1. Pottery   67

 2. Roof tiles   80

 3. Metal objects   81

 4. Other items   86

 5. Summary   90

Chapter VI Interpretations  93

 1. Nature of the Mizuochi Site  93

 2. The open area to the west of the Asukadera Temple and the water clock facility  111

 3. The time-keeping system and the Divination Bureau, Mediate Affairs Ministry  123

 4. Conclusion   139

Supplementary Chapter  Analyses based on the natural sciences  141

 1. Chemical analysis of the residue from the inner wall of a sprue-cup, and a piece of slag (tentative identification)  141

 2. Inferences based on comparative ratios of lead isotopes about the source of raw materials used in the copper pipe recovered from the Mizuochi Site  143

 3. Quality of the material and the structure of the copper piping  151

 4. Hydraulic analysis regarding multi-level water clocks  151

 5. Dendrochronology assessments of the ages of the wooden conduit and trough   158

Supplementary Tables  159

English Summary  161

Textual materials  188(1)

 1. Textual materials related to the history of water clocks  186(3)

 2. Textual materials related to the Shumisen figure and the tsuki tree west of the Asukadera Temple  176(13)





  1 Topography of the vicinity of the site

  2 Archaeological features of the site

  3 Archaeological features (north-eastern portion)

  4 Archaeological features (north-central portion)

  5 Archaeological features (north-western portion)

  6 Archaeological features (middle-eastern portion)

  7 Archaeological features (middle-central portion)

  8 Archaeological features (middle-western portion)

  9 Archaeological features (south-eastern portion)

 10 Archaeological features (south-central portion)

 11 Archaeological features (south-western portion)

 12 Stone paving (northern and southern sides)

 13 Stone paving (eastern and western sides)

 14 Subterranean crossbeams

 15 Lacquered wooden boxes





Frontispiece Aerial view of the site

 1 Panorama of the investigated area

 2 Panorama of the investigated area (top), base stones and subterranean crossbeams (bottom)

 3 Lacquered boxes and wooden trough

 4 Seams of small diameter copper piping

 5 Casing for small diameter copper piping (top), silver solder (bottom)





 1 Aerial view of the Asuka and Fujiwara regions

 2-3 Aerial views of the Mizuochi Site

 4 Aerial views of the Mizuochi and Ishigami Sites



 This report is a compilation of the results of six archaeological excavations, beginning in 1972, undertaken by the Nara National Cultural Properties Research Institute at the Asuka Mizuochi Site, located in Asuka Village, in the Takaichi District of Nara Prefecture.


 The Asuka Mizuochi Site (hereafter abbreviated as the “Mizuochi Site”) lies near the center of the Asuka region, where a succession of imperial palaces were built in the seventh century A.D. Its location borders on Asukadera Temple, the first full-scale Buddhist temple in Japan, and sits to the east of the Asuka River, facing the hill Amakashi-no-oka on the west.


 The Mizuochi Site was first discovered in 1972, during an archaeological investigation (the First Excavation) conducted in conjunction with the construction of a new housing project. It was fully anticipated that important features and artifacts might lie buried at this location, as it lies in the district assumed from ancient times as the site of the Asuka Kiyomihara-no-miya Palace, and also because of the discovery in 1902 of unusual stone carvings, including the Shumisen and the anthropomorphic Sekijinzo figures, just fifty meters to the east. It was for this reason that an excavation was conducted prior to construction, which resulted in the discovery of a unique building foundation, in the shape of a square earthen podium, faced with a stone paving. Based on these results, the housing project was canceled, and in 1976 the site was designated for preservation as a national historic landmark.


 The contents of the Mizuochi Site became clear, in more concrete and detailed fashion, during the Second through Sixth Excavations, conducted from 1981 to 1986 in conjunction with work to prepare the grounds as a historic landmark. In these investigations, a succession of highly unusual features and artifacts to be described shortly were discovered, and as the result of a comprehensive assessment of a variety of data related to them, the Mizuochi Site was concluded to be the remains of a water clock, said in the Nikon shoki to have been built in Asuka by the then Crown Prince Naka-no-Oe in 660, the sixth year of the reign of Empress Saimei.


The main features discovered in the excavations include building SB200, having pillars set on base stones and constituting the center of the site, and the embedded-pillar buildings SB280 and SB180, flanking it to the north and south. In addition, there were wooden culverts A through F, and small diameter copper piping, which run longitudinally and transversely through the platform of SB200, wooden culverts G and H running through SB280, and lacquered wooden boxes placed in the center of SB200.


 The base stones for SB200 formed a grid, with stones at all but the most central point, defining a square floor plan that was four spans long on each side. Each span, the distance between two pillars, was made with a high degree of precision to a uniform length of 2.74m. The building’s podium, 10.95m on a side and slightly more than 1 m high, was paved with stones on all four sides and sloped gently inward. Each of the 24 base stones embedded in the podium had a round seat approximately 40cm across, carved on the top surface for a pillar. After the pillars were erected with their bases inserted into these seats, the surrounding area was tamped firm as part of the process of building up the podium. In addition, as the base stones were linked with one another by embedded stone alignments, serving as subterranean crossbeams, the building was provided with an extremely sturdy structure.


 An immense stone platform, measuring 1.6m east-west and 2.3m north-south, was set near the center of building SB200, at about 1 m below the surface. A shallow depression was carved into the surface, on top of which remnants of one large and one small lacquered wooden box were found. Wooden culverts were buried around the platform. The flow of water, as can be reconstructed from the placements and inclinations of these items is complex, but can be classified into five main water systems. Of particular interest among these is culvert B, extending from the eastern side of the site up to a trough just in front of the lacquered wooden boxes. A large copper pipe, some 3cm in diameter, was inserted at one point along culvert B, from which it evidently extended to the surface of the podium. This was probably a device for raising water from culvert B up to the podium surface, by closing off a trough downstream. The water thus raised to the top of the podium probably collected in the lacquered box on top of the stone platform, and was later drained off to the west by culvert F, which connects to the southwestern part of the platform. When the trough was opened, excess water from culvert B flowed out to the north, via culverts C, D, and E. A separate water system is represented by small diameter copper piping, which extends north from the west side of the stone platform in parallel with culvert E, passing under the stone pavement and continuing further north. This copper piping was probably a device for conveying water from SB200 to another building, located away from it to the north.


 The various characteristics of the Mizuochi Site listed above are completely unique, having no parallel at any other site. For this reason there were many difficulties involved in determining, the site’s nature, but because they were so unusual, a comprehensive assessment of these characteristics led to the water clock hypothesis, as follows. Namely, the sturdy construction of building SB200, made by strengthening the base of every pillar, and the podium’s outer surface made in the shape of a square, were not out of keeping with the Chinese and Japanese practice of placing water clocks atop high stands. Also, as means for bringing water into the podium and drawing it up to the level of the podium’s surface were verified, the existence of some kind of apparatus using water within the building was indicated. In light of this, attention focused on the stone slab buried at the building’s center, and the remains of the lacquered wooden boxes on its upper surface. In particular, a sediment of fine sand left at the bottom of the boxes suggested they were part of a mechanism connected with water. It is noteworthy that the smaller wooden box was contained within the northern half of the larger one The smaller box, lying in the very center of the building, is the site’s pivot and appears to indicate its original function. At 37cm measured internally, the box is nearly the same size as the receiving tank of water clocks known for the Yuan, Ming, and Qing Dynasties. Finally, the results of a simulation, employing modern hydraulics and based on this assumption, demonstrated a high level of precision for this type of water clock.


 The separate water system, consisting of the smaller copper piping, was an apparatus for supplying water which utilized the difference in elevation between the building and ground level, and there is a high possibility that it conveyed water from SB200 to a fountain, similar to the Shumisen or the Sekijinzo figures, at the Ishigami Site. The results of investigations into the chronological age and the nature of the Ishigami Site do not contradict this assumption. At some point in time these two sites functioned as a single unit.


 From an examination of the pottery recovered in the excavations, it is inferred that the Mizuochi Site was built and abandoned within the period spanning the 650s and 660s, a finding in keeping with the age assumed by the above hypothesis. Also, among the iron objects from the site, there are nails with gilded heads and 6-foliate shaped metal fittings; it is very likely that these were used for the lacquered wooden box. Further, the small diameter copper piping, made from copper plate and silver soldered, is very thin at an inside diameter of 0.9cm, indicating the high level of silver soldering technology at the time. By the same token, it is easily conceived that similarly small copper pipes were used as conduits linking the different tanks of the water clock. The small copper piping laid down within and outside the podium was not buried directly in the soil, but enveloped with a lacquer paste and wooden casing, clearly showing a concern for protecting it from the cold and from damage. Also, the copper used in both the larger and smaller pipes represents the oldest example of domestically produced copper, as evidenced by examinations of lead isotope ratios. These are significant results, demonstrating that the Mizuochi Site was built with the most advanced technology at the time.


 As just indicated, the Mizuochi Site is regarded as the locus of a water clock; the primary significance of installing a water clock in Asuka during the reign of Empress Saimei was the implementation, premised upon the introduction in the first half of the seventh century of Chinese calendrical and temporal systems, of a full-scale system of time-keeping, using a flawless clock not subject to changes in the weather or the alternations of night and day. This was nothing less than the concrete realization of the notion, grounded in the Chinese political ideal of keeping time and informing the people, that “Emperor, the Son of Heaven, regulates time.”


1995年3月30日 発行



奈良国立文化財研究所学報 第55冊