PURPOSE: To securely actuate parking brake by installing a valve mechanism connected to the pressure oil supply line to an oil hydraulic clutch, and a linking means which switches over to a state where the supply of pressure oil is made impossible in linkage with the operation of parking brake.

CONSTITUTION: A parking brake is so constituted that a control valve V3 is connected on the upstream side of the oil line a of an auxiliary control valve V2, i.e., the pressure oil supply line to an oil hydraulic clutch C, and the control valve V3 is switched over in a direction to shut off the supply of pressure oil to the auxiliary control valve V2 in linkage with the operation of the operating ever 19 of a parking brake 14. It can be made possible, therefore, when the parking brake 14 is operated, to maintain a changeover valve 17A, of four pilot pressure operating type two-position changeover valves constituting a changeover valve 17, in a shut-off state to fail the supply of pressure oil to the clutch C, i.e., to prevent motive power from being transmitted to driving wheels.