Operator	Chosen Yusen (under the IJA control)
Classification	Cargo ship
Gross tonnage	2,295 tons
Speed
Departure point	Batavia, Java Is.
Departure date	29 July 1944
Destination	Makassar, Celebes Is.
No. of POWs	1,513 (Javanese Romusha)
Location of disaster	Makassar Strait
Date of disaster	4 August 1944
POW casualties	1,239
POW survivors	274
Photo

As the airfield in Makassar became in need for repair, Koshu Maru with the Javanese Romusha aboard and Shinai Maru were escorted by the auxiliary mine layer Keinan Maru and net-layer Hayabusa Maru, and they were crossing the Java Sea at low speed. On 2 August, this convoy arrived at Kotabaru, Laut Island on the southeastern tip of Borneo. As of the following day, with the torpedo boat Kasasagi replacing Hayabusa Maru, they contused the voyage to Makassar. At dawn on 4 August, the USS Ray sighted Koshu Maru in the middle of Makassar Strait, and fired four torpedoes at her. The first torpedo hit the engine room amidships on the starboard side, and two mores struck No. 3 hold in the bow section. The ship broke in two and sank instantly. The people aboard Koshu Maru did not have a chance to escape. Two-hundred and seventy-three passengers, 28 crewmembers and gunners, 1,239 Javanese Romusha went down with the ship. In addition to the Javanese Romusha, Koshu Maru was transporting 540 passengers and air base supplies.