A driving cam includes a circular surface portion at an outer cam surface thereof. When an engine is turned off, the circular surface portion is brought into contact with a roller. Thereby, even if a load is applied to the control axis member, no rotational force is circumferentially applied to the driving cam. As the result, even if a driving force is not applied from a motor to the driving cam, a rotational angle of the driving cam and an axial position of the control axis member are kept constant. Therefore, it is possible to keep the driving cam and the control axis member at a constant position without adding a complicated mechanism.