At the end of 1944, the Imperial Japanese Navy decided it needed large numbers of high-speed coastal submarines to defend the Japanese Home Islands[1] against an anticipated Allied invasion (named Operation Downfall by the Allies).

[1] For surface running, the submarines were powered by a single 400-brake-horsepower (298 kW) diesel engine that drove one propeller shaft.

[2] While she was underway off Etajima on her way to Kure, Japan, for a hull refit on 6 August 1945, the United States Army Air Forces B-29 Superfortress Enola Gay dropped an atomic bomb on nearby Hiroshima.

[2] The detonation of the atomic bomb at 08:15 inflicted severe radiation burns on Ha-204′s bridge crew, including her commanding officer.

[2] Her wreck eventually was salvaged, and it was scrapped between August and October 1948 at the Nishimura Tekkosho ironworks plant.