Delivery ports of a closed circuit hydraulic pump are connected to a head-side chamber and a rod-side chamber of an arm/boom cylinder. A switching valve is arranged between the head-side chamber and a delivery port of an open circuit hydraulic pump. A proportional control valve is arranged between the head-side chamber and a hydraulic fluid tank. At times of cylinder extension, both of the closed and open circuit hydraulic pumps and the switching valve are controlled so that the delivery flows from the closed and open circuit hydraulic pumps are sent to the head-side chamber. At times of cylinder retraction, the closed circuit hydraulic pump and the proportional control valve are controlled so that part of an outward flow from the head-side chamber is returned to the closed circuit hydraulic pump and other part of the outward flow is returned to the hydraulic fluid tank.