PURPOSE: To control the torque constant without using a mechanical feedback mechanism, by generating the capacity of the variable delivery type hydraulic pump for varying the discharge quantity per revolution, i.e., capacity by tilting-revolving a swash plate.

CONSTITUTION: A swash plate 7 is tilting-revolved by the pressurized oil supplied into the control port 37 of a valve plate 34. The spool 55 of a variable control valve 41 is switched between a communication position and a cut-off position by the differential pressure between before and behind a throttle 38 formed in the discharge passage 31 of the hydraulic pump 30, differential pressure between the pump discharge pressure and the load pressure, and the differential pressure before and behind a throttle 63 formed in the discharge flow passage of a fixed pump 66. Further, the spool 65 of the variable delivery valve 41 is switched between the communication position and cut-off position by the variation of the flow rate of the hydraulic pump 30, variation of revolution speed, and the variation of the pump discharge pressure. Accordingly, the capacity of the variable capacity type hydraulic pump 30 can be controlled, keeping the torque constant, without using a mechanical feedback mechanism. Further, the differential pressure between the pump discharge pressure and the load pressure can be controlled constant.