Taiyo was the escort carrier made from the NYK liner Kasuga Maru. Mitsubishi at Nagasaki laid her down on 6 January 1940 and launched her on 19 September 1940. The navy took her to Sasebo for the conversion. She was commissioned on 2 September 1941, still under the liner's name. The flight deck was flush, and she had no arresting wires. She was for ferrying aircraft and training pilots. She could not stay with the fleet. Unyo was a sister.
Before the war she had already carried fighters out to Palau. After the landings on Guadalcanal she sailed with Yamato toward Truk, then was sent off to deliver aircraft. On 31 August 1942 the name was changed to Taiyo. She came into Truk on 4 September.
On 28 September 1942, south of Truk, Trout hit her once, aft. Thirteen men were killed. She made the lagoon, left for Japan on 4 October, and was back in service on 26 October. The ferry runs went on.
On 9 April 1943, bound for Truk, four torpedoes from Tunny struck her and did not explode. She needed no repair. On 6 August, on the way from Truk toward Yokosuka, Pike fired four torpedoes and missed. Onami went after the submarine. On 24 September, northwest of Chichi Jima, Cabrilla hit her on the starboard quarter and wrecked the starboard propeller. Two more torpedoes against the bow did not explode. She lost power for a time. Unyo towed her into Yokosuka on 26 September.
In 1944 the work was the escort run down to Singapore. On 10 August she left Imari Bay with convoy HI-71. On the night of 18 August, off Cape Bolinao, Rasher hit her aft. Fire took the hangar and she listed to starboard. Captain Shuichi Sugino flooded the port magazines. The aviation gasoline aft then exploded, and he ordered the ship abandoned. She sank at 2248, at 18°10′N, 120°22′E.
Sugino was among the saved. The count of the saved is 417, crew and passengers together. How many had been aboard is not settled, so the number of the dead is not a single figure. Of the three liners converted into this kind of carrier, Chuyo had already gone down in December 1943. Unyo was still at sea.