PURPOSE: To form the reference hole of almost complete roundness on the inclined surface part with the punch for round hole of cheap price and the bush for round hole by setting the angle between the seat surface of the inclined surface part and the surface crossing orthogonally to the inserting direction of the reference pin to the prescribed rate of the angle between the blanking direction of the reference hale and the inserting direction of the reference pin.

CONSTITUTION: The punching direction P of the reference 1 hole 2 is formed with the relation of being inclined by the prescribed angle α with respect to the inserting direction Q of the reference pin. The seat surface 3 is formed by standing partially by the angle of the inclined surface toward the surface V side perpendicular to the inserting direction Q of the reference pin with the down edge of the reference hole 2 as the base point at the forming position of the reference hole 2. The angle γ of this seat surface 3 between the surface V perpendicular to the inserting direction Q of the reference pin is set at about 1/2 of the angle α between the punching direction P of the reference hole 2 and the reference pin inserting direction Q. If the seat surface 3 is formed by the angle γ, the shape of the reference hole viewed from the reference inserting direction Q is made almost to the complete roundness. Therefore, the round hole punch and the round hole bush are can be used for punch-forming the reference hole 2.