PURPOSE: To reduce the time required for making a long hole on a metal plate and extend the life expectancy of a male mold, by forming large projections whose depth of concave exceeds thickness of the plate and then cutting off the projections to the level of the plate's face.

CONSTITUTION: A rotor 1, flat magnetic plate, is placed on a female mold 9, its adsorptive face 1a facing a groove made on the mold 9. A projection 11 whose depth L of concave exceeds thickness T of the rotor 1 is made with a pointed and arched male mold 10 on the adsorptive face 1a of the rotor 1. The intermittent arched projections 11 formed on the rotor 1 are then cut off to the level of the adsorptive face 1a. This method can reduce the working time required for making the hole and extend the life expectancy of the male mold.