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). To meet this requirement, the Ha-201-class submarines were designed as small, fast submarines[1] incorporating many of the same advanced ideas implemented in the German Type XXI and Type XXIII submarines. They were capable of submerged speeds of almost .[1]
The Ha-201 class displaced 320LT surfaced and 440LT submerged.[1] The submarines were 53m (174feet) long, had a beam of 4m (13feet) and a draft of 3.44m (11.29feet).[1] For surface running, the submarines were powered by a single 4000NaN0 diesel engine that drove one propeller shaft.[1] When submerged the propeller was driven by a 1250shp electric motor.[1] They could reach 11.8kn on the surface and submerged.[1] On the surface, the Ha-201-class submarines had a range of 3000nmi at ; submerged, they had a range of 105nmi at .[1] Their armament consisted of two 533mm torpedo tubes with four torpedoes and a single mount for a 7.7-millimeter machine gun.[1]
Ha-204 was laid down on 5 April 1945 by the Sasebo Naval Arsenal at Sasebo, Japan, as Submarine No. 4914.[2] She was renamed Ha-204 on 1 May 1945 and was attached provisionally to the Sasebo Naval District that day.[2] Launched on 1 June 1945,[2] she was completed and commissioned on 25 June 1945.[2]
Upon commissioning, Ha-204 was attached formally to the Sasebo Naval District and assigned to Submarine Division 33 in the Kure Submarine Flotilla for workups.[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]
Ha-204 had not yet conducted an operational patrol when hostilities between Japan and the Allies ended[1] on 15 August 1945. She surrendered to the Allies at Sasebo on 2 September 1945.[2] She ran aground in Aburatsu Bay on the coast of Miyazaki Prefecture on Kyushu on 29 October 1945.[2]
The Japanese struck Ha-204 from the Navy list on 30 November 1945.[2] Her wreck eventually was salvaged, and it was scrapped between August and October 1948 at the Nishimura Tekkosho ironworks plant.[2]