Through dividing all blades into four or more blade groups including a certain number, which is three or more, of the blades, the blade located in the rearmost portion of each blade group in a direction of rotation is selected as a main blade, and remaining blades are selected as auxiliary blades, the length of each of the auxiliary blades is set to be shorter than the length of the main blade, and corresponding inner edge portions are positioned to the front, in the direction of rotation, of a normal line that passes through an outer edge portion of the blade, and an extension line of a chord line that connects the outer edge portion and the inner edge portion of the blade to one another are made to intersect with the main blade that is adjacent to the front in the direction of rotation.