A plurality of stepped parts which have step surfaces facing an upstream side in a rotating axial direction of a structural member are provided in a tip portion of a blade, and seal fins which extend toward a circumference surface of each of the stepped parts and form small clearances between the seal fin and the circumference surface corresponding to the each of the stepped parts is provided in the structural member. Also, lengths from the small clearance to the step surface on the upstream side along the rotating axial direction of the structural member are set such that one of the stepped parts on the downstream side is smaller than the other one of the stepped parts on the upstream side.