The motor (50) transmits its rotation to the main shaft (1) through the first transmission gear train (9) while transmitting rotation at a rotating speed ¾ as high as the rotating speed of the main shaft (1) to the eccentric shaft (2) through the second transmission gear train (10) and the offset shaft coupling (7). As the motor (50) is driven, an eccentric axis (30) moves on a circle decentered by a predetermined length L while the inner blade (3) moves in the same direction on a circle further decentered by the predetermined length L from the eccentric axis (30). The inner blade (3) rotates and moves while exposing part of its outer peripheral surface exteriorly of a tube (W) at plural positions on the outer periphery of the tube (W).