The subject of this photo is a Kiraisen minelaying submarine built in 1926 patterned after a first world war German U-boat. This submarine was originally number 48, renumbered I-21 when this photo was taken in 1930, and renumbered I-121 when a new B Type submarine I-21 was built in 1939. B Type submarines had a streamlined seaplane hangar forward of the conning tower where a formation of sailors appears in this image. The seaplane catapult required the gun to be mounted aft of the conning tower in B type submarines, rather than forward like the gun in this image.
{{Information| |Description = 伊号第二十一潜水艦 (I-21 submarine) |Source = The Japanese book "軍艦写真帖" (pictures of warships) a revised edition |Date = in 1930 |Author = published by 海軍協会 (The Navy Association of Japan) |Permissi