An example impeller includes: an impeller body in which an internal channel is formed, the internal channel extending inside the impeller body in a direction of a rotation axis spirally about the rotation axis to connect an inlet and an outlet; and at least one centrifugal vane provided in the impeller body. The internal channel including the inlet and the outlet has a predetermined passage diameter. An external channel is formed so as to continue to the outlet and go around the circumferential surface of the impeller body, the external channel being defined by the centrifugal vane and being recessed inward in the radial direction from the circumferential surface of the impeller body. At least a part in a flow direction of the external channel has a channel width in the direction of the rotation axis smaller than the width of the outlet.