PURPOSE: To reduce the extent of surface roughness on a work surface and prevent bubbles and dust in a sealed resin from sticking to the surface by finishing both the inner and outer circumferential surfaces of a framelike resin dam, made up of punching a substrate material, with a diamond deposited punch and a die.

CONSTITUTION: An inside diameter hole 92 of a resin dam is punched in a substrate material 90 by an inside diameter punch 20 and a die part 52 at a first stage A, while the blanked chips 93 are discharged downward, and a diamond deposited punch 30 is inserted into the hole 92, thus the inner circumferential surface is finished at a second stage B. The punch 30 cuts irregularities on the inner circumferential surface of the hole 92 at an inducing part 34 and finishes a straight part 32. A bore diameter is punched by an outside diameter punch 40 and an outside diameter punching die part 56 at a third stage C, and a separated resin dam 86 is pushed by a tip of the punch 40 and fed to the lower part, then irregularities on the outer circumferential surface are cut out at a diamond deposited inducing part 57, thus it is finished at another straight part 58.