An impeller is attached to a rotary shaft provided in an inner space of a housing and includes a tube fixed to the rotary shaft and a plurality of blades protruding toward an outer diameter direction from the tube. A tip of each blade is in slidable contact with an inner peripheral surface of the housing. Each blade has a shape curved toward a rotation-direction rear side of the rotary shaft in a free state and includes an extension surface on a rotation-direction front side of the rotary shaft and a compression surface on the rotation-direction rear side of the rotary shaft. A curvature radius of a root on the compression surface in the blade is formed larger than a curvature radius of a root on the extension surface in the blade.