PURPOSE: To secure an air control action, when a fluid type retarder device fails and pressure air leaks from one air tank by avoiding lowering of pressure in an other air tank through a process of closing a switch valve and supplying pressure air from a compressor.

CONSTITUTION: When a retarder switch 5 is switched, for example, to (2) position, pressure air from a second air from a second air tank 4 is inducted into the pressure chamber 17c of a pneumatic cylinder device 17 through a first switching valve 31 so that a clutch device 13 may be put in connection. Therefore a supporting member 19 is turned and a rotor 16 integrally formed thereto also turns together with a rotary shaft 10. At that time, a second switching valve 32 stays in a low pressure position (d) and pressure air in the second air tank 4 is supplied to the air chamber 25d of an air-liquid converting device 25 through a first pressure reducing valve 33, resulting in generation of braking torque in a fluid type retarder 11. The switching valve 9 is switched to a closed position (d) by a control unit 100 when the pressure air in the second air tank 4 leaks.