Japan's first postwar submarine “Oyashio” was launched at No. 1 Building Berth at Kawasaki Dockyard on May 25, 1959. Representing the latest fabrication techniques in the construction of all-welded undersea craft, this vessel is now commissioned under the Japanese National Defense Agency. This paper deals with the records of fabricating and welding techniques used for the construction of this craft. Summerized below are the points worthy of special mention : 1) Used for inner hull structure were weldable high tension steels, HT 50 and HT 60, and various quality tests were performed on each of the steel plates used. 2) The so-called unit cylinder assembling system was adopted in order to shorten the period of construction. 3) The length as well as the fabricating sequence of the unit cylinder assembly were decided in accordance with the results of the fabricating tests on models. 4) Many kinds of special jigs were devised in order to maintain the circularity of the inner hull structure and used successfully in the construction.