Abstract: First, welding protrusions of two aluminum alloy plates are butted against each other, and an engagement projection of one of the aluminum alloy plates is brought into engagement with an engagement recess of the other aluminum alloy plate, thereby maintaining the butted state and defining voids in back of butt regions of the welding protrusions in a direction of irradiation of a laser beam. Then, the laser beam is irradiated to the butt regions to bond the welding protrusions to each other. The Al.sub.2 O.sub.3 films located on the surfaces of the welding protrusions on a back side in a direction of irradiation of the laser beam are pushed into the voids by a molten pool and hence, they cannot intrude into a weld zone to form a notch. By employing this arrangement in the butt welding using the laser beam, the need for a jig for maintaining the butted state of the aluminum alloy plates is eliminated and a butt joint having a high strength can be obtained.