A case is formed of aluminum. A rotation axis has one end affixed with an input rotor element and is rotational with a rotor. A first bearing is equipped to the case and is located between the input rotor element and the rotor to rotationally support the rotation axis. A second bearing is equipped to the case and is located on an opposite side of the rotor from the first bearing to rotationally support the rotation axis. A multilayered helical wave washer is wound for three rounds or more in a spiral wave form and is located in an axial gap formed between the case and an outer ring of one of the first bearing and the second bearing to apply a spring load to the outer ring in the axial direction.