PURPOSE: To realize the more precise control of engine speed and to reduce the drop of engine speed at the loaded time by counting the output of an oscillator means making a response to an error signal proportional to a difference between a source input signal and a control output signal and driving an actuator.

CONSTITUTION: A comparator means 27 for comparing the source input signal 3 and the control output signal 26 and generating the error signal 28 proportional to the difference between the source input signal 3 and the control output signal 26 and the oscillator means 29 for making the response to the error signal generating a timing signal 43 proportional to the error signal 28 are provided. A counter means 44 making the response to the source input signal 3 generating the timing signal 43 and a continuous counter output signal 51 and actuator means 52 and 58 making the response to the continuous counter output signal 51 adjusting actual speed are provided. Thus, the speed of an internal combustion engine is automatically controlled and constant speed can be substantially adjusted to be maintained even at the time of the load.