In a fuel tank in which an upper joint flange of an upper tank half body and a lower joint flange of a lower tank half body are sandwiched between and seam welded by a pair of roller electrodes, the lower tank half body includes a lower tank main body portion and expansion tank portions shallower than the lower tank main body portion and disposed on opposite sides of the lower tank main body portion. A side wall of at least one of the upper and lower tank half bodies is provided with roller escape recesses recessed inwardly, the roller escape recesses being formed at a position corresponding to a connection portion of a main body corresponding joint portion and expansion tank corresponding joint portions in a joint portion formed by seam welding, and avoiding a contact with at least one of the pair of roller electrodes.