PURPOSE: To eliminate discrepancy between a set value and an actual value of operating hydraulic pressure by determining the operating hydraulic pressure of a main relief valve through operating some of remote controlled valves by means of electro- magnetic directional control valves provided in by-pass lines of the remote controlled valves connected to a vent of the main relief valve in series.

CONSTITUTION: When an actuator 1 is stopped, electro-magnetic directional control valve 11a to 11h are not magnetized, and by-pass lines 12a to 12h are opened. In this case, working fluid flows through a vent of a relief valve 4 and the by-pass lines 12a to 12h Consequently, the main relief valve 4 is unloaded. When the actuator 1 is driven and some or all of the by-pass lines 12a to 12h are closed, pressurized working fluid pass through remote controlled valves 10a to 10h. Then, differential pressure, which is equal to the operating pressure, is produced before and behind the remote controlled valves 10a to 10h. Consequently, the sum of the said operating pressure is applied to the vent of the main valve 4. By this constitution, it is possible to set the operating pressure of the main relief valve 4, i.e., the operating pressure of the hydraulic circuit, multistagewise.