To prevent concentration of stress from occurring on an area that has had a weld toe by performing an ultrasonic impact treatment.

An after-shock shaft center contact portion/R2 end distance A as a horizontal distance between a contact point 480 with a shaft 440 of a virtual pin 430 in an impact impression and an R₂ end 490 at the point of time when the impact impression is set to be larger than 1.2 mm. A pin tip space B as area of a region among the virtual pin 430, a virtual weld bead 410, and a virtual metal material 420 at the point of time when the virtual pin 430 contacts the virtual weld bead 410 and virtual metal material 420 is set to be equal to or less than 0.2 mm². In a region of the impact impression 240, a stress concentration coefficient ratio C that is a value obtained by dividing an elastic stress concentration coefficient K_t(R₂) of a region, which a second exposure surface 310b contacts, by an elastic stress concentration coefficient K_t(R₁) of a region, which a first exposure surface 310a contacts, is set to be less than 1.75.