PURPOSE: To provide a horsepower control device for variable displacement oil hydraulic pump wherein a several number of hydraulic actuators of differently setting a relief valve can be economically driven by enabling a cut off function to change in two steps as necessary.

CONSTITUTION: In the first switching valve 14 for a flow cut off, provided in a horsepower control device for this variable displacement oil hydraulic pump, a pressure receiving area in one end side of a spool of the valve 14 is formed larger than that in the other end side, and a pressure receiving surface 12c of delivery pressure of area corresponding to a difference between both the pressure receiving areas is formed in a spool of the first switching valve, to connect chambers in both ends of the spool to a tank or pilot pump 16 through the second switching valve. A shutoff valve is provided in a circuit of connecting the pilot pump to the first switching valve. Accordingly, a cutoff pressure can be changed in two steps by operating the switching valve, and by providing the shutoff valve, only horsepower control can be performed by arbitrarily releasing a cutoff function of the shutoff valve.