Found at the foot of Kengoshizuka exactly where the chronicles said it should be — a rare spot where text and archaeology click into place.
Finds & research
Monolithic corridor chamber of quartz diorite; finds not detailed.
Two views on the occupant
Archaeological view
The mound is levelled and its measured size is unknown, though an Asuka Village study suggests a possible reconstruction as a square mound about 20m on each side. Its megalithic side-entry chamber of quartz diorite — hollowed out of a huge block and opening to the south — was found in 2010. Built in the late 7th century, its position "before" Kengoshizuka (id:266) matches the Nihon Shoki's 667 record of Princess Ota's burial fronting Empress Saimei's tomb, so the leading view identifies it as the tomb of Princess Ota (daughter of Emperor Tenji and mother of Prince Otsu) — the discovery that decisively strengthened the Kengoshizuka-as-Saimei theory.
Imperial Household Agency view
Not designated by the IHA. Part of the National Historic Site "Kengoshizuka & Koshitsuka-Gomon Kofun" (1923; this mound added 2014).
Field notes
Maintained with Kengoshizuka (id:266); free to walk through, with the chamber viewed through a fence and paid reserved openings. While Kengoshizuka was inscribed as a component of the World Heritage site "Ancient Capitals of Asuka and Fujiwara" in 2026, Koshitsuka-Gomon itself is not among the 19 listed components.