"Musashi" was commissioned at Nagasaki on 5 August 1942, and assigned to the 1st Battleship Division together with "Yamato", , and . Beginning five days later, the ship conducted machinery and aircraft-handling trials near Hashirajima. Her secondary armament of twelve 127 mm guns, 12 triple 25 mm gun mounts, and four anti-aircraft machine guns was fitted from 3–28 September 1942 at Kure, as well as a Type 21 radar. The ship was working up for the rest of the year. Arima was promoted to rear admiral on 1 November.