Nihonkai Maru carried the name of the Sea of Japan. The characters are 日本海丸. Mitsui Bussan's shipbuilding department laid her down at the Tama yard on 6 October 1931, launched her on 28 March 1932, and completed her on 30 April 1932. Shimatani Kisen owned her. The characters of the company are 嶋谷汽船. Her official number was 37467 and her call sign was JKNC. The minesweeper-tender register puts her at 2,681 gross tons. The tender list that follows her officers prints 2,861. uboat.net, in the account of the submarine that sank her, prints 2,684. That tender list groups her with the ships of 朝海丸.
The navy took her up at Sasebo on 22 July 1938 and registered her on 25 July as an auxiliary minesweeper tender. She left that register on 25 May 1939. On 15 November 1939 she came back as an auxiliary torpedo-boat tender. From 2 December she was with Sentai 13 in the 3rd Fleet. On 27 November 1941 she went to the Shanghai Base Force, on 15 February 1942 to the 3rd Expeditionary Fleet, and on 10 April 1942 to the Tsingtao Base Force.
Captain Nakatsu Seiki had her from 1 July 1941. The confidential personnel gazette of 28 November 1941, issue 758, names him as her captain in the order that moved him on. Captain Endo Masaru's dates on the tender list start that same day and run to 7 August 1942. One column of that list still prints Nakatsu through 7 August, overlapping Endo. Yoshizo Nakamaruo had her from 7 August 1942 until 1 October 1943. That morning she was reclassed an auxiliary transport, a miscellaneous ship, and he was attached to the Kure Naval District. The captain's column ends there. On the same list the chief engineer, reserve lieutenant Imafuku Masayoshi, runs on to 20 October.
She stayed under the Sasebo Naval District until the loss. One account of the last voyage has her bound from Yap toward Davao. On 17 June 1944 Flounder (SS-251) sank her. James Edward Stevens had the submarine. The Japanese register prints the place as 06°36′ N, 127°55′ E, in the Philippine Sea. The tender list prints 06°33′ N, 127°55′ E, and calls it 118 miles off Cape San Augustin. Twenty of her crew are counted dead. She was removed from the navy list, and released, on 10 August 1944.
She is not the sail-training ship Nippon Maru, and not the cruise ship of that name. A photograph of this hull did not turn up.