First, a nozzle insertion process in which a nozzle is inserted into a water cooling hole having a small diameter which is provided in a back of a mold is performed. Then, an injection process in which a mixed flow of air having a pressure ranging from 0.1 MPa to 1.0 MPa with an injection particle is jetted from a leading end of a nozzle to a distal portion of the water cooling hole. As a result, a shot peening process is performed on the distal portion of the water cooling hole.