To weld and join a polygonal first shape to a second shape by using a manipulator, while moving a torch at high speed.

A first shape 1 and a second shape are welded and joined. After forming a radius on the corner part 1b of the first shape 1, welding is performed while moving a welding torch 3 held by a manipulator at high speed along the peripheral direction of the first shape 1. The welding torch 3 is moved in the corner part 1b of the first shape 1 along a virtual circular arc provided on the corner part 1b, and the welding is performed while changing successively the position of the welding torch 3.