PURPOSE: To reduce power loss of an engine by providing a hydraulic clutch connected only in the time of braking between a gear pump absorbing engine load and a shaft driven by the engine.

CONSTITUTION: When a brake pedal of a vehicle is depressed a switch 31 is closed for opening an electromagnetic switchgear 34 to operate clutch plates 39 with hydraulic pressure in an oil gallery to transmit rotation of an input shaft to a gear pump via a gear wheel 22. In this case, an electromagnetic closing valve 35 is closed. Oil in an oilpan 2 is transferred to a clutch 23 by a pump 7. On this time, the electromagnetic closing valve 34 is closed with signal from an oil pressure sensor 9. The hydraulic clutch 23 is connected with hydraulic pressure in the oil gallery, about 5kg/cm2, on the early stage of braking operation, but when the gear pump 7 is driven, the clutch 23 is connected with this output pressure (about 200kg/cm2 maximum) therefore braking capacity is increased. When braking operation is released, the hydraulic clutch 23 is released, too, and the pump 7 also is not driven, therefore there is no loss in engine power in usual running.