PURPOSE: To improve manufacturing and processing efficiency without requirement on formation of a mounting unit having a stopper with bolt and nut on each blade by forming recessed grooves on a main plate and a side plate so as to fitting and fix each blade in each recessed groove with a binder.

CONSTITUTION: Respective components like a main plate 1, a side plate 2, blades 3 and so on are formed by a fibre reinforced plastic material. On the main plate 1 and the side plate 2, the recessed grooves 11, 21 for inserting the blades are formed on the positions corresponding to the blade mounting parts on the outer circumference parts thereof. The fitting parts 31, 31 corresponding to the recessed grooves 11, 21 are formed on both ends of the blades 3. Notches 32 for preventing their falling off are formed as occasion demands on one or both of the side parts of these fitting parts 31.