PURPOSE: To facilitate mounting performance onto a vehicle by leading negative pressure into the vacuum chamber of an air-liquid converter to generate pressure liquid in the first and second hydraulic chambers of a first and a second hydraulic cylinders, and power-boosting a second hydraulic plunger member in relation to a first hydraulic plunger member to drive a clutch device.

CONSTITUTION: When a retarder switch is turned on to supply a vacuum chamber 23c with negative pressure from a negative pressure supply source, a pneumatic and a first hydraulic plunger members 23b, 23e are integrally put in going action so as to generate pressure liquid in a second liquid pressure chamber 24c, and a second hydraulic plunger 24b is power-boosted and put in going action. With this action, a pressure plate 14a is thrusted in to connect a clutch device 13, so that a rotor 6 is rotated to display the function of a retarder device. When the retarder switch is turned off to lead atmospheric air into the vacuum chamber 23c, the pneumatic and first hydraulic plunger members 23b, 23e are integrally reset by a return spring 23h, and the second hydraulic plunger member 24b is also reset to cut off the clutch device and thereby to stop the rotor 6.