The was a ship class of "cruiser submarines" of the Imperial Japanese Navy (IJN). There were four submarine designs of the Junsen type: J1, a modified J1, J2 and the J3. "Junsen" is the abbreviation of .
The Junsen type submarines were divided into four subtypes:
Four boats were built in 1923-1929. Genealogy of the large-size submarine in the IJN began with . Japan received six U-boats from Germany as reparations of World War I. The IJN copied , one of the six, producing the I-21-class minelayer submarine. The IJN could not find an optimal design of fleet submarine, so they and Kawasaki Heavy Industries sent many technical officers to the United Kingdom and Germany and got drawings of advanced submarines. The British L class became the Kaidai I, the K class became the Kaidai II, and U-142 became Junsen I.
This is a type which added a floatplane to the Junsen I.
Project number S32. This is a type which added a catapult to the Junsen I Mod. She was built in 1931 under the 1st Naval Armaments Supplement Programme (Maru 1).
Project number S33. These boats combined the good points of the Junsen II and the Kaidai V. They were built in 1934 under the Maru 2 Programme. Junsen III became a 'typeship' for the Type-A, B and C.