To hold a blade without working a groove in a blade so as to restrict the positional displacement of the blade in the thrust direction due to the centrifugal force at the time of rotation by specifying the fitting angle of support pulleys.

A straight line for connecting the centers of an inside support pulley 7 and an outside support pulley 8 vertically crosses a virtual center shaft 9, and an inside pulley 7' and an outside pulley 8' are similarly arranged. These straight line form an angle at 60-80 degree around the virtual center shaft 9. In the case of the angle less than 60 degree, blade position holding accuracy is insufficient, and displacement at the time of rotating the blade one time can not be restricted. Positional displacement of the blade 1 in the thrust direction by the centrifugal force of the rotation thereof is restricted and the blade is guided by the fitting angle of the support pulley, and the rotation of the blade 1 is thereby held.