To provide a means to give high fatigue strength to a metal product.

A closed space 8 is formed on a treating surface 1a of the metal product 1 so that a surface opposed to the treating surface 1a becomes a circular arc surface 2a, and a work material 5 constituted by including a globular body or a polyhedral body the specific gravity of which is 2 to 10 and the diameter of which is more than 5 mm and less than 20 mm exists, and the work material 5 1a is made to collide against the treating surface 1a by oscillating it in a direction parallel with the surface direction of the treating surface 1a.