PURPOSE: To deviate an exfoliating point on the rear side of a blade toward the rear edge to lessen the exfoliating region by locating the maximum warping position of a warping curve interconnecting the front and rear edges of blade toward the rear edge from the center between the front and rear edges.

CONSTITUTION: A plurality of blades 12 are disposed on the outer peripheral surface of a hab 1 configured into a generally circular truncated cone. The section of blade 12 is configured such that a position (b) of the warping center line 17 of blade having the maximum height from a straight line interconnecting the front and rear edges 13, 14 is deviated from the central portion 15 toward the rear edge side. Since the maximum warping position is deviated thus toward the rear edge, the exfoliating point of flow on the rear side is deviated toward the rear edge to reduce a turbulence region 16 on the blade surface.