A side plate of an impeller is formed so that the height thereof is lowered gradually from an edge portion of a central opening portion toward a circumferential portion. When the distance from the edge portion of the central opening portion to the circumferential portion, in a direction perpendicular to the output shaft, is taken as L, the distance from the edge portion of the central opening portion to the circumferential portion, in the direction of the output shaft, is taken as H, a point on the side plate away from the edge portion of the central opening portion by 0.1×L in the direction perpendicular to the output shaft is taken as P, and the distance from the edge portion of the central opening portion to the point P in the direction of the output shaft is taken as ΔH, ΔH/H≧0.4 is satisfied. With this configuration, the formation of a vortex flow in the flow channel inside the impeller from the air inlet to the air outlet is reduced, and, thus, air blowing efficiency is improved.