PURPOSE: To prevent cavitation by interposing a pressure reducing valve in a communicating pipe line between a pilot valve of a remote control valve end a pilot pressure receiving section of a pilot change-over valve and causing load pressure of a hydraulic actuator to act on the pressure reducing valve to adjust the secondary pressure.

CONSTITUTION: Pilot valves 7a, 7b of a remote control valve 5 are connected to a pilot pressure receiving section 9R through a pilot pressure receiving section 9L of a pilot change-over valve 8 and a pressure reducing valve 20. The load pressure side pressure of an arm cylinder 4 is introduced into a pilot port 21 of the pressure reducing valve 20. Thus, when the bottom side pressure of an arm cylinder 4 reaches predetermined low pressure, the secondary pressure of the pressure reducing valve 20 as pilot pressure of the pilot change-over valve 8 drives spool to prevent the occurrence of cavitation without relating to the pilot pressure of the pilot valve 7b of the remote control valve 5.