A fuel tank comprising: an upper tank half body that is made of metal and press formed so as to have an upper joint flange on an outer periphery of the upper tank half body; and a lower tank half body that is made of metal and press formed so as to have a lower joint flange on an outer periphery of the lower tank half body, the upper joint flange and the lower joint flange being sandwiched between and seam welded by a pair of roller electrodes (56, 57), characterised in that the lower tank half body includes a lower tank main body portion and a pair of expansion tank portions that are shallower than the lower tank main body portion and disposed on opposite sides of the lower tank main body portion, the upper joint flange and the lower joint flange are seam welded together to form a joint portion that includes a main body corresponding joint portion and a pair of expansion tank corresponding joint portions, the main body corresponding joint portion being disposed on an outer periphery of the lower tank main body portion except for an area between the joint portion and the expansion tank portion, the pair of expansion tank corresponding joint portions being connected to the main body corresponding joint portion and disposed on an outer periphery of the pair of expansion tank portions except for an area between the joint portion and the lower tank main body portion, and a side wall of at least one of the upper tank half body and the lower tank half body is provided with a roller escape recess (61, 62) that is recessed inwardly, the roller escape recess (61, 62) being formed at a position corresponding to a connection portion of the main body corresponding joint portion and the expansion tank corresponding joint portions, and configured for avoiding a contact with at least one roller electrode out of the pair of roller electrodes (56, 57). an upper tank half body that is made of metal and press formed so as to have an upper joint flange on an outer periphery of the upper tank half body; and a lower tank half body that is made of metal and press formed so as to have a lower joint flange on an outer periphery of the lower tank half body, the upper joint flange and the lower joint flange being sandwiched between and seam welded by a pair of roller electrodes (56, 57), characterised in that the lower tank half body includes a lower tank main body portion and a pair of expansion tank portions that are shallower than the lower tank main body portion and disposed on opposite sides of the lower tank main body portion, the upper joint flange and the lower joint flange are seam welded together to form a joint portion that includes a main body corresponding joint portion and a pair of expansion tank corresponding joint portions, the main body corresponding joint portion being disposed on an outer periphery of the lower tank main body portion except for an area between the joint portion and the expansion tank portion, the pair of expansion tank corresponding joint portions being connected to the main body corresponding joint portion and disposed on an outer periphery of the pair of expansion tank portions except for an area between the joint portion and the lower tank main body portion, and a side wall of at least one of the upper tank half body and the lower tank half body is provided with a roller escape recess (61, 62) that is recessed inwardly, the roller escape recess (61, 62) being formed at a position corresponding to a connection portion of the main body corresponding joint portion and the expansion tank corresponding joint portions, and configured for avoiding a contact with at least one roller electrode out of the pair of roller electrodes (56, 57).