To lower the maximum Mach number caused by the acceleration of a back side of a blade, to weaken the shock wave and reduce the pressure loss, by making a radius of curvature of a back side blade face of specific % of a chord length from a front edge, smaller than a radius of curvature of the back side blade face from the specific % of the chord length to a rear edge.

In the maximum thickness position 16, a radius of curvature of a back side blade face 19 to a position of 50% of a chord length 15, is 0.6 times of a radius of curvature of a back side blade face 1A from a position of 20% of the chord length to a rear edge 12. The inflow air current 1 is temporarily decelerated by a front edge 11, then is accelerated on the back side blade face 19 between the front edge 11 of small radius of curvature and 20% of the chord length 15, and reaches 1.2 equal to the inflow Mach number near the front edge. Then the inflow air current exists in an area 1A of large radius of curvature of the back side blade face, so that the acceleration degree is reduced, the maximum Mach number is regulated to be 1.3 or below, and the total pressure loss caused by the shock wave can be reduced.