To reduce a quick stop torque when a rotor blade is broken without upsizing a casing member.

A lower cylinder part 4a is formed in an inner peripheral side of a rotor blade 6a at the lowermost step of a rotor 4 housed in a casing member 12. A cushioning member 20 against which the chips of the rotor blade 6a collide when the rotor blade 6a is broken is coupled to a base 13, or integrally arranged with the base 13, in the outer periphery side of the lower cylinder part 4a, thereby a timing when the impact of chips not colliding against the cushioning member 20 is transmitted to the casing member 12 and a timing when the impact of chips colliding against the cushioning member 20 is transmitted to the casing are shifted.