Koza Naval Arsenal

The Koza Naval Arsenal (高座海軍工廠) was one of the Imperial Japanese Navy's key aircraft production facilities during World War II. In 1942, the Navy Ministry ordered the construction of a large-scale aircraft factory in the Koza District of Kanagawa Prefecture, spanning parts of what are now the cities of Zama, Yamato, and Ebina. The facility was designed with an ambitious target capacity of 30,000 workers and 6,000 aircraft per year, though the deteriorating war situation prevented these goals from being achieved.

Construction began in 1943 and the arsenal was formally completed in 1944, primarily producing the Mitsubishi J2M Raiden ("Thunderbolt") interceptor fighter, designated to defend the Japanese homeland and the Imperial Palace against Allied bombing raids. Between 1944 and 1945, the Koza facility manufactured a total of 128 Raiden aircraft — 17 in 1944 and 111 in 1945 — accounting for roughly one-fifth of Japan's total Raiden production.

One of the most distinctive and historically significant aspects of the Koza Naval Arsenal was its workforce. Facing severe labor shortages, the Japanese military recruited approximately 8,400 Taiwanese youths (台湾少年工, Taiwan Shōnenkō) aged 12 to 19. Selected for academic excellence, physical fitness, and loyalty to the empire, these boys were transported from Taiwan to Japan beginning in 1943. They were housed in forty two-story wooden dormitories in the Kamikusa area north of Yamato Station. Though nominally receiving technical training, they were effectively conscripted industrial laborers. Some were dispatched to other aircraft factories across Japan, while others remained at Koza to build the Raiden.

As the threat of air raids intensified following the fall of Saipan in July 1944, the Navy ordered the construction of underground factory tunnels in the hills of Zama. Five large underground chambers were excavated in the Serizawa area, with a combined tunnel length of approximately 1,500 meters. These subterranean facilities were intended to protect aircraft component production from Allied bombing.

After Japan's surrender in August 1945, the arsenal was dismantled. The former factory grounds were gradually converted to residential and commercial use. Today, the underground tunnels survive beneath Serizawa Park (芹沢公園) in Zama, and portions are occasionally opened for public viewing. In October 2018, a memorial stele honoring the Taiwanese youth workers was erected in Serizawa Park, marking the 75th anniversary of their arrival in Japan. The monument bears poems exchanged between the former Taiwanese workers and Japanese women who had served alongside them. Annual commemorative events maintain the friendship between Japanese and Taiwanese communities connected to this wartime history.