To provide a method for carrying out a peening to a welded structure, the method enabling a remarkable improvement in the fatigue strength of a weld joint by introducing compressive residual stress by applying, with a usage of a simple tool, a peening treatment to the welded structure after a welding.

A method for carrying out a peening is provided, the method enabling an improvement of fatigue performance of a weld joint by striking the surface of a mother material by using a peening tool 2 comprising a striking tool 1 having a flat part on its tip. The striking is to be carried out with the striking tool 1's striking direction being inclined to the opposite side to the welded part at 5 degrees or more and at 20 degrees or less with respect to the direction perpendicular to the surface of the mother material.