A unit for controlling a variable capacity type hydraulic pump using an engine as a driving power source. This unit is provided with a means (5) for detecting the number of revolutions per minute of an engine, a means (6) for detecting the discharge pressure of a hydraulic pump, a means (12) for setting on the basis of the number of revolutions per minute of the engine the suction torque characteristics of the pump, which decrease monotonously with respect to this number of revolutions, a means (7) for determining an angle of pivotal movement of a swash plate in the pump on the basis of the suction torque characteristics and discharge pressure of the pump, a means (8) for controlling the swash plate in the pump so that this angle of pivotal movement of the swash plate of the pump can be attained, and means (7, 9, 10) for reducing the number of revolutions per minute of the engine on condition that the suction torque of the hydraulic pump does not exceed the allowable torque of the engine. According to this control unit, the number of revolutions per minute of the engine can be reduced to a lowest possible level, and a cost-per-mile can be minimized. Moreover, the angle of pivotal movement of the swash plate in the pump can be increased to a highest possible level, and the operational efficiency of the pump is improved.