PURPOSE: To eliminate the need for stretching work by MIG welding right and left plate materials from the inside surface side and building up the weld beads in a crest shape on the inside surface of the tank.

CONSTITUTION: Butt end edges 11a, 12a of pair of right and left plate materials 11, 12 of a drawing board shape are MIG welded from the inner side of the tank to form the plate materials 11, 12 into one piece, whereby the tank main body 10 built up with weld beads 3 in a crest shape on the inside surface at the weld zone 10a is obtained. Part 13a of the beads just protrudes on the outside surface of the tank main body 10 and this is easily polished away by a polishing device 14, whereby the tank main body 10 is completed. Since the weld beads 13 are formed only on the inside surface side of the tank 10, there is no need for roller leveling, hence there is no need for plateworking correction work for stretching. Thence, a bottom plate 15 is joined to the tank main body 10 by means of low frequency seam welding, whereby the fuel tank for motor bicycles is obtained.