Shōmonzuka, Ancient burial mound in Ōtemachi, Japan.

Shōmonzuka is a kubizuka, a mound said to hold the head of Taira no Masakado, in Ōtemachi, Chiyoda, Tokyo. A tall light-grey stone stele with a rounded top edge rises behind a low retaining wall of rough-cut greyish-brown blocks capped in flat grey; its face is carved vertically with 都旧跡 将門塚. A dense green hedge and a pale plaque on a short post stand to the left.

The site emerged in the 10th century after a samurai led an uprising against imperial power. When the rebellion ended, his head found its final resting place here, and the spot later became a place of veneration.

The name connects to Masakado, whose spirit receives honor through small shrines and offerings left by visitors. Office workers from nearby buildings stop by regularly, keeping an old custom alive in the middle of the business district.

The mound stands immediately by exit C6a of Ōtemachi Station on the Tokyo Metro and Toei subway lines. Since the 2020–2021 refurbishment, offerings, donated objects and the incense burner are off-limits inside the enclosure; only coins may be placed in the offering box carved with the nine-star crest. Offerings for Masakado are accepted at the Kanda Shrine office in Soto-Kanda.

The site survived several attempts to relocate or remove it, because accidents and misfortune were linked to disturbing the grave. That is why the grounds remain today in their original spot amid high-rise office buildings.