National Treasures & Important Cultural Properties
of National Institutes for Cultural Heritage, Japan

There was once an island called Takeshima, located in the Seto Inland Sea off the coast of Shūnan City in Yamaguchi Prefecture. Only 600 meters across, Takeshima joined the islands of Kurokami and Ōzu in forming Tokuyama Bay. Ōzu was the training base for pilots of the manned torpedoes developed and used during the Second World War. Since then, part of the bay has been reclaimed, joining Takeshima to the mainland. Located at the top of Takeshima is a keyhole-shaped burial mound named the Takeshima Gokarō Yashiki Tumulus. Measuring about 56 meters long, it dates from the early Kofun period (4th–6th century). As it is currently situated on private property, access is restricted.

In 1888, this tumulus was unearthed during an attempt to develop the land. Inside, the landowners discovered mirrors and weapons in what was likely a keyhole-shaped tumulus. Since then, the finders passed down these artifacts through successive generations of their family. Eventually, in the late 1970s, one of the excavated objects, a bronze mirror with a triangular rim depicting deities and beasts, caught the attention of researchers who found an inscription on it, indicating an emperor's reigning era name used in the Chinese state of Wei. The year of this inscription corresponds to the year 240, when Chinese envoys were sent to Japan after a Japanese group had been dispatched to China by a queen named Himiko. This mirror could have been one of the “hundred bronze mirrors” mentioned in historical accounts as gifts sent to Himiko by the emperor of Wei.

Similar mirrors have been unearthed from various kofun in other parts of Japan, including the Kanisawa Tumulus in Gunma, the Morio Tumulus in Hyogo, and the Sakurai Chausuyama Tumulus in Nara. Another triangular-rimmed mirror from the same tumulus, also depicting deities and beasts, shares its design with those from the Tsubai Ōtsukayama Tumulus in Kyoto, the Kannokura Tumulus in Fukuoka, and the Kase Hakusan Tumulus in Kanagawa. This set of precious artifacts can contribute greatly to our understanding of how mirrors had been distributed by the Yamato polity.

Researchers at the Takeshima Tumulus have also excavated weapons like iron swords and knives, iron and bronze arrowheads, and tools such as iron axes. All artifacts from this tumulus are collectively designated as Important Cultural Properties by the Japanese government. As a prime example of ancient grave goods from the early Kofun period, they are highly evaluated for their research value.