A blade included in an impeller is formed so that, when viewed in a vertical cross-sectional view of the blade, a pressure surface of the blade and a suction surface of the blade opposite to the pressure surface are curved more in the direction in which the impeller rotates, in their areas farther from the axis of rotation of the impeller and closer to the exterior of the blade, and are arched so that a portion near the center of the blade is most distant from a straight line connecting the inner end and the outer end of the blade, the pressure surface and the suction surface form a curved surface including at least one circular arc, and a straight portion of the blade is formed to be connected to the curved surface on its one side, and extend toward the inner end of the blade on its other side.