PURPOSE: To hold a differential pressure between pump discharge pressure and the maximum load pressure always at a value determined by a pump regulator even in a slow speed region of an actuator.

CONSTITUTION: Pressure in a maximum load pressure line 9 of an actuator 6 is introduced into a pump regulator 10 and an discharge amount of a pump 1 is controlled by the pump regulator 10 in a load sensing hydraulic circuit. While a pimp discharge pressure line 2 is connected through a throttle 21 and pressure reducing valve 22 to a maximum load pressure line 9, the pump discharge pressure line 2 is connected to a tank 30 through a controlling valve 23 changed over to the open side (communicating side) by the pressure in the maximum load pressure line, a flow adjusting valve 24 and flow changing-over valve 25.