Rotational speed of an electric turbocharger controlled so that the actual temperature of the motor does exceed the allowable level even over a long time span. A speed control device for the turbocharger, includes a temperature sensor, a speed limitation device which limits the speed of the motor in response to the temperature level detected by the temperature sensor wherein the speed limitation device is provided with a limit control start temperature correction device which decreases the limit control start temperature when the increase rate of the detected temperature exceeds a threshold. A speed limitation setting device sets a speed limit based on the temperature difference between the limit control start temperature and the detected temperature T on a rate of increase of the detected temperature.