First, it is determined in a determination step by a determination unit whether there is a nitrided layer on a surface of a water-cooled hole of a mold, by using an eddy current sensor. Next, in a shot step, when it is determined in the determination step that there is no nitrided layer, the surface of the water-cooled hole of the mold is shot-peened under a shot condition set according to a base material of the mold, and when it is determined in the determination step that there is the nitrided layer, the surface of the water-cooled hole of the mold is shot-peened under a shot condition which maintains a state where there is the nitrided layer.