Upper and lower shell members of aluminum-plated steel panels have respective flanges overlapping each other and welded to each other by a laser beam, thereby forming a fuel tank. One of the flanges has a stepped region spaced from the other of the flanges, thereby defining a gap between the stepped region and the other flange. The laser beam is applied to the flanges closely to the gap for discharging unwanted components such as aluminum from a welded area of the flanges into the gap. Therefore, the amount of aluminum contained in the welded area is reduced to increase the bonding strength of the welded area.