A welding method for making cladding or buttering on an inner surface of a base material, an inner surface of an opening portion formed in the base material, and the cut surface formed in such a manner that the cut surface is continuous from the inner surface of the base material to the inner surface of the opening portion, wherein the welding method includes a step of forming a protruding portion on the base material in advance, the protruding portion including a temporary welding surface extending toward the center of the opening portion in such a manner that the temporary welding surface is uniformly continuous to the inner surface of the base material and including the cut surface buried therein.