PURPOSE: To prevent a cut burr from protruding from a fin, by forming a surface in the end part of a lift blade to a curved arc shape while providing aside a notched part corresponding to a fixed blade and cutting the corrugated fin in its part displaced from the center of a flexed part of said fin.

CONSTITUTION: A cutter, forming a surface in an end part 4a of a lift blade 4 to a curved arc shape, provides a notched part 4b in a position slightly upper from said part 4a, and an external angular part of the notched part 4b forms an edge point 4c corresponding to an edge point 3a of a fixed blade 3. A fin 1, being positioned on the fixed blade 3 in a position displaced from the center of a flexed part 1a, is cut by the blades 3, 4. The cutter, producing cut end parts 1b, 1c of the fin in a little inner position, prevents the cut end parts from protruding to the outer from the external end of the fin even for cut deformation and burr generation.