There is provided a method for reducing the nonuniformity of forward voltage Vf of an N-type semiconductor wafer in which density of impurities included in an N-layer is nonuniformly distributed in a plane view of the semiconductor wafer. The method reduces the nonuniformity of forward voltage, by irradiating charged particles to the N-type semiconductor wafer, and generating defects in the N-layer to reduce the nonuniformity of forward voltage. In one aspect of the method, charged particles are irradiated so that a reaching positon in a depth direction or an irradiation density may differ according to the density of impurities in the N-layer in the plane view of the semiconductor wafer.