Hirai Castle was located on top of the cliff on the left bank of the Ayu River, which runs through Fujioka City, Gunma Pref. It is said to have been built in 1438 by Uesugi Norizane, the Kanto Kanrei (the responsible head of the shogun’s executive office in the Kanto region). It was captured by Hojo Ujizane of Odawara and then regained by Uesugi Kenshin, who, however, removed his base to Maebashi and dismantled Hirai Castle. The excavation researches have been carried out since 1997 and relics of the mid-15th century to the late 16th century were dug out. Earthworks, moats, virtical moats, and the ruins of the bases for bridge retainers, hottate-bashira (earthfast post) strucures, pit dwelling, and toilet were excavated. Recognized its historical value, the castle ruin is designated as a prefecture’s historic site.