PURPOSE: To provide sufficient performance with a simple system, and to assure safety in cases of faults of main sensor or actuator by preventing the runaway by means of the actuation of a limiter when the opening of a throttle valve exceeds a certain level, and by outputting an alarm signal by means of the detection by a sensor for the space between cars and by an obstacle sensor.

CONSTITUTION: When there is a fault in a controller 11 in a slow running mode and the order is given to open a throttle valve 6 to an extraordinary level, if the opening of the throttle valve 6 reaches the maximum value that is preliminarily set, a limiter 21 is actuated to cut off the power source of a motor 22 and to restore the throttle valve 6 to its totally enclosed position. An alarm display device 10 is actuated by the controller 11 based on the detection output of a sensor for the space between cars 9 and of an obstacle sensor 20, to give the alarm.