An automatic speed control device for a vehicle or a stationary generator for emergency use is disclosed. The device comprises a speed signal detector means for generating speed pulses of a frequency proportional to a speed, a constant time generator means connected to the detector, a speed pulse counter means for counting the speed pulses received within the constant time, an auxiliary pulse generator means for generating auxiliary pulses of a constant period from the leading edge of each speed pulse, an auxiliary pulse counter means for counting from the leading edge of each speed pulse auxiliary pulse of constant period sufficiently smaller than a period of the speed pulse at the highest speed, an actual speed computing means connected to both counters for computing an actual speed from the counted result of speed pulses at the end of the constant time and the counted result of auxiliary pulse, a setting speed memory means connected to the actual speed computing means, a speed comparator means for comparing the actual speed and the setting thereby to generate an output signal corresponding to the difference thereof, and an electromechanical transducer means connected to receive the output signal of actual speed computing means for driving an adjustable speed mechanism of a prime mover.