PURPOSE: To enable a constant speed operation by obtaining difference between a real speed, an acceleration obtained from the real speed and a given speed command, and correcting advance notching, unchanged notching and reverse notching or a speed suppression brake command, according to the magnitude of the difference.

CONSTITUTION: A real speed is obtained with a speed calculating part 41, from a pulse signal inputted in constant speed operation control equipment 4. From the change of speed, an acceleration is calculated 42 and inputted in a speed band judging part 43. Difference between a speed command from the outside and the real speed is obtained every period of a timer 46, with the judging part 43. According to the magnitude of the difference between the speed command and the real speed, the judging part 43 judges which a speed band belongs to out of advance notching, unchanged notching, reverse notching speed suppression braking and emergency braking. On the basis of the judged result, engine notch and brake notch are corrected via an engine notch advance step control part 44 and a brake notch advance step control part 45. As a result, stable constant speed operation free from undershoot and overshoot is enabled with a small number of times of notch switching.