She was laid down on 14 January 1940 at the Innoshima, Hiroshima shipyard of Osaka Iron Works (大阪鉄工所, Ōsaka Tekkosho) for the benefit of Nippon Suisan K.K.[2] She was launched on 3 October 1940, completed on 15 November 1940, and registered in Tokyo.[2] She was the last of 13 Shonan Maru No. 1-class ships built 1938-1940 all sharing the same name (Shonan is the Japanese name for Singapore).[4] The ships of the class in order of completion were: Shonan Maru No. 1 (第一昭南丸), Shonan Maru No. 2 (第二昭南丸), Shonan Maru No. 3 (第三昭南丸), Shonan Maru No. 5 (第五昭南丸), Shonan Maru No. 10 (第十昭南丸), Shonan Maru No. 11 (第十一昭南丸), Shonan Maru No. 6 (第六昭南丸), Shonan Maru No. 7 (第七昭南丸), Shonan Maru No. 8 (第八昭南丸), Shonan Maru No. 12 (第十二昭南丸), Shonan Maru No. 15 (第十五昭南丸), Shonan Maru No. 16 (第十六昭南丸), and Shonan Maru No. 17 (第十七昭南丸).[4]
On 26 September 1940, she was requisitioned by the Imperial Japanese Navy.[2] On 16 December 1940, she was designated as an auxiliary submarine chaser and her conversion was completed at the Hikoshima shipyard of Osaka Iron Works on 8 January 1941.[2] She was assigned to the Sasebo Naval District, Third Fleet, as part of the 52nd Subchaser Division (along with netlayer Fukuei Maru No. 15 and subchaser Takunan Maru No. 5).[5][6] The division was attached to the First Base Force based at Takao, Formosa.[5][6] Her commanding officer was Reserve Lieutenant (J.G.) Yada Takeo (矢田武男).[7]
She was attached to the Lamon Bay occupation force (21–23 December 1941) serving as an escort to 20 Imperial Japanese Army transports carrying 7,000 troops of the 14th Army; and Imperial Japanese Navy transports Hakusan Maru (carrying the 1st Quartermaster Ports and Docks Unit and 1st Naval Signal Unit), Kimishima Maru (carrying the 1st Naval Guard Unit), Senko Maru (carrying the 1st Naval Survey Unit), and Myoko Maru (carrying the 1st and 2nd Sasebo Special Naval Landing Forces).[5] The convoy’s escort consists of Shonan Maru No. 17, light cruiser Nagara, heavy cruiser Ashigara, six destroyers (Tokitsukaze, Yukikaze, Kawakaze, Suzukaze, Umikaze, Yamakaze), minelayer Aotaka, two minesweepers (W-7, W-8), auxiliary gunboat/minelayer Ikumshima Maru, four auxiliary gunboats (Busho Maru, Keiko Maru, Kanko Maru, Myoken Maru), one auxiliary netlayer (Fukuei Maru No. 15), and one auxiliary subchaser (Takunan Maru No. 5).[5] After the successful occupation, she returned to Kaohsiung on 6 January 1942 and then proceeded to Legazpi which had been seized by the Japanese on 12 January 1942.[2] She departed on 16 January 1942 escorting a small convoy consisting of transport ships Kenyo Maru (乾洋丸), Tokyo Maru (東京丸), and Giyu Maru (義勇丸) arriving Davao on 18 January 1941.[2] She then departed for Bangka Island, just east of Sumatra, arriving on 24 January 1942.[2]
