A first stage blade (12) adopting an axial entry method is provided that has an aerodynamically optimal airfoil profile (61) obtained by improving a conventional airfoil to eliminate flow separation from the ventral side of a blade surface. In the present invention, an airfoil profile (61) is formed such that a passage width ratio, represented in a dimensionless manner, of an inter-blade passage width of vain root cross-section to an outlet throat width substantially monotonously decreases from a blade inlet toward a blade outlet.
