Former Army Kaikōsha (Kudan Kaikan Terrace)

The former Kudan Kaikan (九段会館), originally named Gunjin Kaikan (軍人会館, Military Personnel Hall), stands at 1-6-5 Kudan-minami, Chiyoda-ku, Tokyo, adjacent to Yasukuni Shrine and the Shōwa-kan museum. Constructed as part of the Shōwa Imperial Enthronement Commemoration Project (昭和御大礼記念事業), the building was completed on March 20, 1934. Its working design was by architect Ryōichi Kawamoto (川元良一), chief engineer of the Dōjunkai Housing Association, based on Ono Takeo's prize-winning competition entry, in the distinctive Imperial Crown Style (帝冠様式) — a fusion of reinforced-concrete Western construction with a traditional Japanese castle-style tiled roof.

The Kaikōsha (偕行社), established in 1877, was the Imperial Japanese Army's officers' club and social institution, headquartered in the Kudan area. The Gunjin Kaikan served as a broader facility for the Imperial Reservists' Association (帝国在郷軍人会), providing assembly halls, accommodation, and banquet facilities for active and reserve military personnel.

On February 26, 1936, young Imperial Japanese Army officers launched a coup d'état attempt known as the February 26 Incident (二・二六事件). The Martial Law Headquarters (戒厳司令部) was established in the Gunjin Kaikan on the following day, February 27. Lieutenant General Kashii Kōhei, the Tokyo Garrison Commander, was appointed Martial Law Commander and directed operations from the building. The famous radio broadcast "To the Soldiers" (兵に告ぐ), which called on the rebels to stand down, was drafted in the second-floor command room and transmitted from the building's projection room.

During World War II, the building continued to serve military functions. After Japan's surrender in August 1945, the Supreme Commander for the Allied Powers (GHQ/SCAP) requisitioned the building in September 1945, renaming it "Army Hall" and using it as quarters for Allied military personnel.

The building was de-requisitioned in 1953 and subsequently transferred under a free-use agreement to the Japan War-Bereaved Families Association (日本遺族会), which renamed it Kudan Kaikan (九段会館). For decades it operated as a hotel, wedding venue, and conference hall, becoming a familiar Tokyo landmark.

On March 11, 2011, the Great East Japan Earthquake caused the auditorium ceiling to collapse during an event, killing two people and injuring many others. Kudan Kaikan ceased operations in April 2011. In September 2019 the building was entered in the national register of tangible cultural properties.

A major redevelopment project by Tōkyū Land Corporation preserved the northern and eastern L-shaped portions of the original Imperial Crown Style façade while constructing a new 17-story (approximately 75 meters tall) office tower behind it. The complex reopened as Kudan Kaikan Terrace (九段会館テラス, KUDAN-KAIKAN TERRACE) in October 2022, housing offices, conference and banquet facilities, a shared workspace, and commercial spaces.