Kongosan Maru was a cargo and passenger ship. The characters are 金剛山丸. Uraga Dock laid her down on 30 August 1927 for Chosen Yusen, launched her on 21 December, and completed her on 19 February 1928. She was registered at Jinsen, the port of Inchon, at 2,116 gross tons. She ran from the Hanshin ports to Busan, the east coast of Korea, and Vladivostok. She was 280 feet between perpendiculars, with one triple-expansion engine on one shaft, and she made a little over 13 knots.
In 1938 she was measured at 2,119 gross tons. On 30 January 1940 Nihonkai Kisen took her over, and her home port became Tokyo.
The navy requisitioned her on 4 December 1940 and registered her as an auxiliary gunboat on 16 December, in the Maizuru Naval District. A reserve lieutenant, Yamazaki Kiyochi, was her first commanding officer. Mitsubishi Kogyo finished the conversion at Kobe on 20 January 1941. She was rated a gunboat on 5 February. She had come into the year attached to the 2nd Gunboat Division, as an auxiliary minelaying gunboat.
Through the spring and summer of 1941 she worked off south China. On 31 July she left that division for the Yokosuka guard. Captain Kawana Takeo took command on 1 October. By December she was with the Ise Bay defense unit. In the first days of January 1942 she laid the fifth and sixth minefields on the exits from the bay. That spring she swept off the Kii Peninsula, Daiozaki, and Ise Bay.
On 4 May 1942 she was on the patrol line off Daiozaki. Trout sighted her at 1226 and took her for a freighter of about 5,000 tons. A stern torpedo at 1328 missed. The next one hit forward of the bridge. She sank in two minutes at 33°32′N, 136°05′E, about 2.7 nautical miles southwest of Inohana, in Kumano, Mie. Nine of her crew were lost. For the next six hours aircraft hunted Trout and dropped eleven depth charges. None of them came close.
She was taken off the navy list on 15 May 1942. She is not the battleship Kongo, and not the armed merchant cruiser Kongo Maru.