PURPOSE: To upgrade the working efficiency by controlling the discharge flow rage of the variable displacement pump exclusive for turning the hydraulic motor with a regulator according to the discharge pressure of the pump.

CONSTITUTION: Variable displacement pumps P1, P2 and P3 are driven simultaneously with a prime mover E. The pump P3 exclusively used for turning is connected to a turning circuit by way of a circuit 24 and a normal/reverse switching valve 25. A brake control valve 34 is provided between normal side feed circuits 26 and 27 of a turning motor M3 and reverse side feed circuits 28 and 29. In the course of circuits 26 and 29, circuits 35 and 36 for drawing the pressure thereof are connected to a low pressure selection valve 37. A choke valve 41 is provided in the circuit 40 while a pilot circuit 42 for actuating the conpensator valve 18 is connected between the choke valve 41 and the low pressure selection valve 37. The pump P3 is controlled by way of a regulator 43.