PURPOSE: To enable a work to be continued, upon failure of the first pump, by providing a flow dividing selector circuit at a circuit for supplying pressure oil to a hydraulic pilot operation valve and each hydraulic actuator respectively from the first and second hydraulic pumps.

CONSTITUTION: A circuit is provided to control each hydraulic control valve V for a plurality of hydraulic actuators with respective hydraulic pilot operation valves PV. Then, the first hydraulic pump P1 is installed for supplying pressure oil to each operation valve PV. Thereafter, the second hydraulic pump P2 is installed for supplying pressure oil to each control valve V. Furthermore, the circuit of the pump P2 is fitted with an oil flow passage selector valve 14 via a flow dividing valve 13. When the pump P1 is out of order, the valve 14 is selected to supply pressure oil from the pump P2 to each operation valve PV, and an excess of the oil is supplied to the actuators. According to this construction, an existing pump can be operated to cope with a pump failure, if any.