A force multiplier (5) configured to assist movement of the operating rod (22) in its axial direction is provided. An engagement recess (52) provided on an outer periphery of the operating rod (22) has a cam surface (52a) configured to make engagement with an engagement ball (51). There is provided a support hole (53) configured to allow movement of the engagement ball (51) in a radial direction of operating rod (22) and to restrict movement of the engagement ball (51) in the axial direction of the operating rod (22). A pressing member (54) configured to press the engagement ball (51) is provided, and the pressing member (54) has a force-multiplying surface (54a).