In a first step, friction stir welding is performed, while supplying a filler into a joining portion, in order to prevent the coarsening of crystal grains of metal materials in a subsequent fusion processing. Then, a fusion processing such as MIG welding is performed. The filler to be added to the joining portion includes a substance that does not chemically react with the metal materials and has a melting point higher than a melting point of the metal materials and a substance in which a misfit with the metal materials is within ±15%. As a result, the coarsening of crystal grains by the fusion processing is inhibited and the joint strength can be increased.