To prevent the occurrence of gap liable to be caused in end parts of a welding surfaces, and to excellently weld the whole welding surface when the end parts are diffusion-welded to each other.

In this method, when a welding system in which end faces of metals are butted to each other through an insert are heated and raised in temperature above the melting point of the insert, the compression force orthogonal to the welding surface is applied to the welding system by a plurality of cycles for a short time, a part in the vicinity of the welding surface is plastically deformed, and the end faces are closed attached to each other through the insert to achieve the excellent liquid phase diffusion welding of the whole end faces. By repeatedly applying the compression force for a short time, the butted metals are prevented from being misaligned in the thickness direction.