To provide an impeller mounting structure capable of adjusting alignment of an impeller and an inner face of a casing even without preparing various adjusting parts.

A cylindrical mounting seat 16, an elastic spacer 35, the impeller 6, a hexagon nut 36 for adjusting set length, and a hexagon nut 23 are mounted in a mounting part 13 of a motor supporting shaft 11. The elastic spacer 35 is provided to ensure a clearance between the impeller 6 and the inner face of the casing 1. The hexagon nut 36 for adjusting set length is screw-fitted into a male screw part 14 of the motor supporting shaft 11 and is tightened to the impeller 6 to ensure and adjust a clearance d. Since the elastic spacer 35 is elastically deformed by tightening force of the hexagon nut 36 for adjusting set length, the impeller 6 is vertically displaced in accordance with tightening force of the hexagon nut 36 for adjusting set length to enable stepless alignment adjustment easily.