A diffuser apparatus includes a diffuser communicating radially outwardly with an exit of an impellor. The apparatus includes a housing with a plate that is rotatable about a rotational axis and forms a hub side wall. A side surface of the plate has a vane having one circumferential end fixed to the plate; the other end is movable in the direction of the axis. The housing includes an inclination path with one end opened to the hub side wall to bendably guide the vane so that the other end of the vane moves in the circumferential direction upon rotation of the plate and moves in the direction of the axis to a projected position to project into the diffuser. An actuator rotates the plate to drive the vane between the projected position and a retracted position where the other end is retracted into the hub side wall.