The ship was requisitioned by the Imperial Japanese Navy (IJN) in early 1941 and was converted into an escort carrier.

Taiyō was initially used to transport aircraft to distant air bases and for training, but was later used to escort convoys of merchant ships between Japan and Singapore.

The ship was torpedoed twice by American submarines with negligible to moderate damage before she was sunk in mid-1944 with heavy loss of life.

[2] Kasuga Maru was the last ship of her class and was built by Mitsubishi Shipbuilding & Engineering Co. at their Nagasaki shipyard for NYK.

Jentschura, Jung and Mickel state that Kasuga Maru was towed to Sasebo Naval Arsenal for conversion on 1 May 1941.

[9] The ship was equipped with six 12-centimeter (4.7 in) 10th Year Type anti-aircraft (AA) guns in single mounts on sponsons along the sides of the hull.

[9] The ship also received a Type 13 air-search radar in a retractable installation on the flight deck at that time.

[5] Shortly after Kasuga Maru arrived at Rabaul on 11 April, the harbor was bombed twice, although the ship was not damaged in the attacks.

Upon receiving news of the American landings on Guadalcanal on 7 August, Kasuga Maru and the battleship Yamato, escorted by a pair of destroyers, together with the 2nd and 3rd Fleets sailed from the Inland Sea bound for Truk.

Taiyō and Chūyō, escorted by two destroyers departed Truk, bound for Yokosuka, Japan, on 16 April.

[6] In December 1943, Taiyō was assigned to the Grand Escort Command and she began a lengthy refit at Yokohama that completed on 4 April 1944.

Eight days later, off Cape Bolinao, Luzon, Taiyō was hit in the stern by a torpedo fired by USS Rasher.

The hit caused the carrier's aft avgas tank to explode, and Taiyō sank 28 minutes later at coordinates 18°10′N 120°22′E / 18.167°N 120.367°E / 18.167; 120.367.