PURPOSE: To improve fuel consumption efficiency by arranging a flow control valve in a supply side hydraulic piping at delivery side of an oil pump and setting relief pressure for idling engine rotation.

CONSTITUTION: A flow control valve 21 is arranged in a supply side hydraulic piping 18 in a hydraulic circuit for a hydraulic operating device at delivery side of an oil pump 11, and sets the relief pressure to an idling engine rotation so as to suppress fluctuation of hydraulic pressure in the circuit caused by variation of oil flow delivered from the oil pump 11. A clutch control valve 13 is arranged in the upstream of a throttle 22 for setting the hydraulic pressure in the circuit and varies the hydraulic pressure (control pressure) in the clutch hydraulic circuit which is ensured through the flow control valve 21, the throttle 22 and a throttle 24 for setting clutch oil spray pressure linearly. Consequently, pressure necessary for engaging the clutch completely is attained and a hydraulic clutch 140 is controlled linearly.