Eccentricity is not allowed to occur in a blade section, and a blade section is maintained in a stable installation state. A hook section 124, which is composed of a shaft section 125 and an end section 126 provided with a corner section 132 that protrudes as a ledge farther to the outside in the radial direction than this shaft section 125, is formed in a blade section 120. A hook hole 144, which allows the passage of end section 126 of hook section 124, is formed in a bolt section 140. Engaging sections 150 are formed around hook hole 144. Corner sections 132 of end section 126 of hook section 124 are engaged with engaging sections 150 when blade section 120 and bolt section 140 are relatively moved in the direction in which they move away from each other in the direction of an axis O in the state in which blade section 120 is rotated by about 60null towards the front in a direction of screwing rotation S with respect to bolt section 140 after inserting and passing end section 126 of hook section 124 through hook hole 144.