PURPOSE: To prevent occurrence of such burrs as project from a surface by leading a soft foaming material between compressing fixtures, and compressing the soft foaming material from both sides with the compressing fixtures to release compression from the foaming material to make through holes at a position serving as the compressing part of the soft foaming material.

CONSTITUTION: A section serving as the compressing part of a soft foaming material 4 is compressed strongly by a compressing fixture 3 provided with protruding compressing part 2 on a surface, so that the appropriate central part of the compressed foaming material are especially crushed to change into a film. Afterwards, by releasing compression, the foaming material parts which have been only pressed to compress begin to recover elasticity, however, the filmed part does not recover elasticity, so the filmed part is torn by the restoring force of its surrounding parts, forming a through hole 5. It is thus possible to prevent burrs from projecting approximately vertically on the surface of the foaming material 4 and machining chips made during drilling form occurring.