PURPOSE: To bend in a prescribed shape a knife for punching a paper sheet, etc.

CONSTITUTION: The knife is held by a supporting mechanism 8 and a pressing mechanism 9 against the upper surface of a fixed base 7. The edge part of a processing die 24 constituting a bending mechanism 10 is abutted on the side face of the opposite side from bending the knife by a vibrating operation while a prescribed holding time and a feeding time are alternately repeated. Each time such bending is performed, the knife is fed by a feeding mechanism 11 from the supporting mechanism 8 to the direction of an arrow 27 and bent at a prescribed radius of curvature. By this process, the elongation of a material in accordance with a plastic deformation at the time of processing is absorbed by a curved part, and a spring back is also suppressed.