The Junsen type submarine was a ship class of submarines of the Imperial Japanese Navy (IJN). There were four submarine designs of the Junsen type: J1, a modified J1, J2 and the J3.
The 2nd Naval Armaments Supplement Programme otherwise known as the "Circle Two" Plan was the second of four expansion plans of the Imperial Japanese Navy between 1930 and the start of World War II.
Construction of the cruiser Tone was authorized under the "Circle-2 Plan"