To facilitate exchange of a blade, by fixing an auxiliary stop member on which a screw adjustable to the screw arranged on a spindle shaft is arranged, on a blade whose shape is standardized, making a blade assembly, and performing detaching with the shape of the blade assembly.

A hub 18 is attached on a blade 17 whose shape is standardized. A mutually engaging part 19 is arranged on the hub 18. A mutually engaging part 32 of an auxiliary stop member 31 is forcibly inserted in the mutually engaging part 19. A mutually engaging part 33 of the auxiliary stop member 31 is mutually engaged with a mutually engaging part of a spindle shaft, an screw 34 of the auxiliary stop member 31 is screwed with a screw of the spindle shaft, and detaching is performed with a holding part 35. Thereby an automatic exchange system of a blade can be realized with simple constitution wherein a blade which is widely used now can be used as is.