The STO-class submarine, also known as the Sen Toku-class, Toku Gata-class, or Sensuikan-class (literally special class submarine), was a group of submarines used by the Imperial Japanese Navy during World War II. They were the largest used during the war, and could carry three seaplanes underwater in a large hanger. Eighteen were planned but only two were completed. These survived the war but were captured by the US and sunk as targets in 1946, without entering combat.


The STO-class boats were 400 feet long with a maximum beam of 40 feet. Surface displacement was 3,900 short tons. Unrefulled cruise range was 37,500 miles.[1] Incorporated into the hull, forward of the conning tower, was a 120 foot long hanger capable of carrying three aircraft. Armament consisted of eight bow tubes with twenty torpedoes, as well as a number of deck guns.[2]


Design work for the STO-class began in early 1942, with construction starting in February 1943. I-400 was completed in late 1944, with I-401 following by the end of the war. Two other submarines in the class, I-403 and I-404, were destroyed by American bombers while being built.[3]

Following completion of the trials, I-400 and I-401 were joined by I-13 and I-14 to form a submarine task force, under the command of Captain Tatsunoke Ariizumi. The flotilla's Aichi M6A1 attack seaplanes were intended to attack the gates of the Gatun Locks of the Panama Canal, before returning to the submarines to allow their crews to be recovered.[N 1] The aircraft themselves would be abandoned after ditching next to the subs, as the pilots would jettison the floats after take off, in order to increase their dive speed for the attack.

Diverted to attack the the anchorage for US aircraft carriers at Ulithi Atoll, I-14, I-400 and I-401 altered course tom rendezvous off Ponape Island in the Eastern Carolines, following the loss of I-13, 630 miles east of Honsu, on 16 July 1945. After the end of hostilities, the giant submarines made their way to Yokosuka Naval Base, at which point Captain Ariizumi committed hari-kiri, to attone for the disgrace of surrender. After being scrutinized in detail by the US Navy, the subs were used as naval gunfire targets.[N 2]

Ships in class


  1. Capyain Ariizumi persuaded the Japanese High Command to attack the Gatun Locks, rather than launch a strike against the US mainland, as an attack on the lock gates would do more damage to key equipment and US morale.[4]
  2. Of the Aichi M6A1 Sieran floatplanes, only one survived. Found virtually intact in a bombed out hanger after the war, this aircraft is currently on display at the National Air and Space Museum in Washington.[5]


  1. Treadwell, Terry C. Pages 117-118
  2. Treadwell, Terry C. Page 118
  3. Treadwell, Terry C. Strike From Beneath the Sea - A History of Aircraft -carrying Submarines. Tempus Publishing Ltd. 1999. ISBN 0 7524 1704 5. Page 117
  4. Treadwell, Terry C. Page 120
  5. Treadwell, Terry C. Page 122

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