PURPOSE: To reduce the rigidity of a filler metal, and to prevent a damage caused by a stress concentration, by butting a high strength material to a low strength material, and melting in the filler metal and executing a joining so that a step difference between both the materials is joined slowly to a groove which has been formed between both the materials.

CONSTITUTION: An end face of a high strength material 2 having a necessary wall thickness (t) is butted to an end face of a low strength material 1, and a joint is formed by melting in a filler metal so that a step difference of both the materials is joined slowly to a groove which has been formed between both the materials. In this way, a stress which is generated on a joint surface part is not concentrated in the low strength material 1 whose rigidity is low, and dispersed. Therefore, a damage of the low strength material 1 is prevented.