Abstract: A hydraulic pressure system comprising a hydraulic pump, a hydraulic motor, a pair of main hydraulic pressure lines connecting the pump and the motor so as to form a closed loop or circuit, a booster line for supplying a hydraulic pressure from a booster pump to the low pressure side of the closed circuit, a pilot line for taking out a pilot pressure from the high pressure side of the circuit to operate an actuator for controlling the hydraulic pump and the hydraulic motor, and a boost pressure regulator for causing the pressure in the booster line to rise only when the differential pressure between the two main hydraulic pressure lines of the closed circuit becomes lower than a predetermined level.