To provide a tool for shock-plastic working and a working method with which a fatigue-strength of a corner part existing at the edge part of a metal is improved.

In the enlarged diameter-state tool for shock-plastic working treatment, whose outer diameter is gradually enlarged from a head part 8 toward a tip end part; the tip end part 5 surface is formed as smooth curved surface and the shape of a side surface from the head part 8 to the tip end part 5 surface is formed as the enlarged diameter so as to become an exponential function-shape. At this time, the shape of the side surface 2 from the head part 8 to the tip end surface can be formed as the enlarged diameter so as to become the exponential function-shape, or can be formed so as to optimize the radius of the curvature on the tip end surface.