To advance the penetration of a joint part and to raise the joining strength of a welding part by chamfering a corner of an end of a backing member and forming a large groove on the end of a plate like member in the case of butt welding of the plate like member of aluminum alloy using the backing metal of aluminum alloy.

A corner part of an end part 61 of a backing member 6 is executed with chamfering 611, and the end part 61 is made orthogonally in contact with the face 21 of a member 2. An end part 51 of a member 5 forming a single bevel large groove 511 is made along the backing member 6 and arranged in a suitable root interval. Then, in this state, the chamfering part 611 is executed with MIG welding. On the obtained welding part, its deposited metal is penetrated to the depth. That is, the complete penetration is enough reached. Therefore, the joining force of the welding part of the members 2, 5, 6 is increased in comparison with the conventionality, and the joint having the enough reliability in the strength can be formed.