Koei Maru was a small cargo ship of 863 tons, built by Amasaki Dock for Amakasu Sangyo. She was launched on 29 May 1941 and completed on 20 September 1941. In the last months of that year the navy took her up and fitted her as an auxiliary net tender. She went to the Fourth Fleet and worked at Truk.
On 21 September 1942, south of the lagoon, Trout fired three torpedoes from the stern tubes. The first broke her in half. The next two hit the after half. She sank near 06°54′N, 151°51′E.
She was taken off the navy list on 20 October 1942. A much larger minelayer carried the same name and was still in service in 1945. This was the net tender.