PURPOSE: To uniform the pressure that a support applies to cutting blades by forming a shoulder on the inner wall above the cutting section of each cutting blade in opposition to the other and forming an upper-end inward section offset to the outer wall by the width of the shoulder on the inner wall.

CONSTITUTION: Cutting blades 1 and 1' having a sawtooth cutting section 2 and opposed to each other in mirror symmetry are each formed with a shoulder 3 above their cutting sections 2 on their inner walls 4 opposed to each other inside their outer walls 5. The shoulders 3 engage with the bottom edge of a support frame to distribute the pressure the support frame gives to the shoulders 3 during cutting to stock uniformly. The cutting blades 1 and 1' are further each formed with a groove-shaped recess 6 in which a fixing tool is fitted to fix and guide the cutting blades 1 and 1' to the support frame.