To surely prevent the deflection of a blade during a cutting operation by mounting a set ring having an abutment portion adapted to be brought into abutment with the upper surface of the blade and having an inner wall forming a width slightly larger than the width of a blade mounting portion in a notched portion provided at a lower end portion of a plunger to function as a blade locking portion.

A knob bolt 6 is loosened that is provided on a plunger 1 in such a manner as to intersect an axial center of the plunger 1 at right angles, and a blade 5 is inserted into a notched portion 3 having a width slightly larger than the width of a blade mounting portion from a lower end direction of the plunger 1. When the knob bolt 6 is tightened, the blade 5 is held between a presser member 4 and a side wall 3a of the notched portion 3, whereby it is retained within the notched portion 3. When the blade 5 is inserted into the notched portion 3, the upper surface of the blade 5 and a side thereof confronting at right angles a cutting direction of a raised portion of the mounting portion are regulated by an abutment surface and an inner wall, this preventing the blade 5 from rotating about a tip end of the knob bolt 6 during a cutting operation. This makes it possible to mount various types of blades having mounting portions each different in configuration and size, thereby making it possible to surely prevent the deflection of the blade 5 during a cutting operation.