PURPOSE: To improve fastening of a blade on a simple manufacturing condition by making a conical screw have a conical head part which is eccentrically abutted on a fastening surface and making fastening force have radial direction and axial line direction components working on a cutting blade.

CONSTITUTION: Fastening screws 5 to be fastened to a rotor main body 1 are provided with conical fastening surfaces 6 and these fastening surfaces 6 cooperate with recessed fastening surfaces 7 with which blades 3 are combined. Therefore, all the blades 3 are provided with the corresponding recessed fastening surfaces 7 on which the conical parts of the fastening screws 5 are abutted at both the end parts. When one of the conical screws 5 is fastened, frictional force imparted to the fastening surfaces 7 combined by the conical surfaces 6 substantially works on the radial inside and has a desired direction wedge action. Because this fastening force has also axial line direction components of the blades 3, the blades 3 are held at definitely stipulated locations by the conical screws 5.