PURPOSE: To enable to prevent overspeed operation of a main engine in a reliable manner even at the time when the level of the load applied to the engine is changed from high to low, by stopping operation of an integrator when a high load is applied to the engine.

CONSTITUTION: In case of controlling the speed of rotation of a main engine to keep it at a constant value near the maximum engine speed, the engine speed is lowered when the load becomes maximum, and the output of a speed regulator is raised to a level higher than the maximum fuel injection rate (100%) so as to keep the engine speed at a prescribed value. Under such circumstances, the fuel injection rate is limited to the maximum fuel injection rate by the function of a stopper. Further, in case that the level of a control signal produced from an operational controller 8 is higher than the level of an upper-limit signal produced from a fuel supply rate determining means, operation of an integrator is stopped and the output signal of an arithmetic means 11 is kept at a constant level. Here, since other arithmetic means 10, 12 for executing PID control are kept in operation, load is lowered again and the output of the speed regulator is brought back to the maximum level promptly.