PURPOSE: To prevent the deformation of a pipe by providing a pair of tip blade parts opposing point symmetrically on the punch main body, forming two pairs of lead blade parts with the tip end blade part as the apex and arranging the groove part for insulation between the lead blades.

CONSTITUTION: A pair of tip blade parts 12, 13 are provided at the tip of a punch main body 11 and two pairs of lead blade parts 14, 15, 16, 17 descending toward the side part respectively with those as the apex are formed as well. The groove part 18, 19 for insulation are respectively arranged between the lead blade parts 14, 17 and 15, 16. When a piercing punch 5 is descended to a set pipe 1, the tip blade part 12 is first cut by a cutting blade 20, the blade part 13 is then cut. In succession, the lead blade parts 14W17 are cut in order. In this case, the loading of a load is prevented on the same place in parallel to the axial line 37 of the pipe 1. Consequently the deformation of the pipe 1 is prevented.