Location and Characteristics of the Mound
The enormous burial mound lies in western Okayama City, where a section of a hill on the right bank of the Ashimori River was cut away to create its distinctive shape. The mound was built in three tiers, with the top of the rectangular front section rising into a raised platform.
The mound measures approximately 350 meters in length, making it smaller only than several of Japan’s largest imperial burial mounds, including the Daisenryo Kofun in Sakai, Osaka Prefecture, at approximately 486 meters, and the Konda Gobyoyama Kofun at approximately 420 meters.
Six smaller burial mounds, known as baichō, are located around the main mound. These include Sakakiyama Kofun, also called Tsukuriyama No. 1 Kofun, and the decorated Senzoku Kofun, or Tsukuriyama No. 5 Kofun. Their interiors have not been fully excavated.
Historical Significance
The scale of Tsukuriyama Kofun suggests that the ancient Kibi region was once home to a powerful political authority, often referred to as the Kibi polity. Its size indicates that this regional power may have rivaled the Yamato polity, which came to dominate much of ancient Japan.
Many of Japan’s largest burial mounds in the Kinki region are managed by the Imperial Household Agency and are generally closed to public entry. Tsukuriyama Kofun is an exceptional example of a monumental ancient tomb that visitors can explore directly, making it particularly valuable for understanding Japan’s early history.
Scale and Structure of Tsukuriyama Kofun
Dimensions of the Mound
Total length: Approximately 350 meters.
Rear circular section: Approximately 190 meters in diameter and 29 meters high.
Front rectangular section: Approximately 215 meters wide and 25 meters high.
The front section was partly destroyed, and Ara Shrine, which serves the Tsukuriyama settlement, now stands there. The shrine’s stone water basin incorporates the main body of a hollow, boat-shaped stone coffin made from Aso tuff. It remains exposed to the weather. Behind the shrine are fragments of a stone coffin lid, traditionally said to have been brought from Shinjo Kurumazuka Kofun.
Structure of the Rear Section
Paths leading to the summit and possibly to the burial chamber are thought to have existed in the past. However, the mound was used as a fortified position toward the end of the medieval period, and much of its original structure was altered or reconstructed.
The summit of the rear circular section is relatively flat, while its slopes are particularly steep. The mound was constructed in three tiers, each with sloping surfaces. The flattened summit is believed to have been modified during the Azuchi-Momoyama period, when forces aligned with the Mori clan may have prepared the area as a military position during Toyotomi Hideyoshi’s campaigns against the Mori.
Discovery of the Projections and Moat
The burial mound is thought to have originally featured projecting platforms on both sides, although only the left-side projection remains today. Located alongside the rectangular front section, it measures slightly over 10 meters wide, approximately 30 meters long, and just over 2 meters high.
For many years, researchers were unable to confirm the existence of a surrounding moat. However, a preliminary excavation conducted by Okayama University in 2010 established that a moat had once encircled the mound.
The Smaller Burial Mounds and Construction Period
Features of the Associated Mounds
Six smaller burial mounds are located around Tsukuriyama Kofun. One notable example is Sakakiyama Kofun, or Tsukuriyama No. 1 Kofun, which is thought to be either a circular mound or a smaller keyhole-shaped tomb. Artifacts discovered there include a horse-shaped belt hook and a bronze mirror decorated with animal and mythical motifs.
Senzoku Kofun, also known as Tsukuriyama No. 5 Kofun, is a three-tiered scallop-shell-shaped burial mound. It contains an early example of a horizontal stone burial chamber, an important feature in the development of ancient Japanese burial architecture.
The associated mounds may not all have been built at the same time as the main tomb. Whether each should be classified as an associated burial mound remains a subject of academic discussion.
Estimated Date of Construction
Based on the production dates of haniwa clay figures found in the area and the mound’s present form, Tsukuriyama Kofun is believed to have been built in the first half of the 5th century.
The tomb traditionally identified as the burial place of Kibitsuhiko, a figure appearing in Japan’s early chronicles, is generally associated with Nakayama Chausuyama Kofun, approximately five kilometers to the east across the Ashimori River. Tsukuriyama Kofun is thought to have been constructed later.
Preservation and Archaeological Research
Current Condition and Excavations
Because parts of Tsukuriyama Kofun have been damaged or destroyed, it is difficult to reconstruct its complete original appearance. Since 2009, an archaeological team from Okayama University has conducted investigations to clarify the mound’s construction date and structure, including searches for artifacts that can help establish its chronology.
Excavations on the eastern side of the rear circular section confirmed the presence of the surrounding moat, providing important new evidence about the original design of this monumental tomb.
Conservation Issues at Senzoku Kofun
Senzoku Kofun, one of the smaller mounds associated with Tsukuriyama Kofun, has faced significant conservation challenges. In 2009, researchers discovered that ritualistic patterns painted on the stone partition inside its burial chamber were flaking away. White mold was also found inside the chamber in 2010.
In response, the Okayama City Board of Education decided to remove the affected stone partition for preservation. Conservation measures similar to those used at Takamatsuzuka Kofun and Kitora Kofun have been considered to protect these fragile archaeological remains.
Access to Tsukuriyama Kofun
By Train and Taxi
From Bitchu-Takamatsu Station on JR West’s Kibi Line, the site is approximately a 12-minute taxi ride away.
By Bicycle
Bicycle rentals are also available near Soja Station and in front of Bitchu Kokubunji Temple, offering an alternative way to explore the surrounding historical sites and countryside.
Tsukuriyama Kofun is a remarkable place to experience the immense scale of ancient Japanese burial architecture firsthand. Combined with nearby archaeological sites and cultural attractions, it offers visitors an opportunity to discover the history of the ancient Kibi region and its influential rulers.