PURPOSE: To prevent the danger such as reckless driving of a car when a constant-speed traveling apparatus is broken, by reducing the number of revolution of an engine by controlling the number of times of ignition or the ignition timing.

CONSTITUTION: When a throttle cable is not returned to the original state, and the number of engine revolution is not reduced, even through the release signal for constant-speed traveling control is transmitted, a control unit 10 judges the generation of a trouble in an actuator 9, if the signal showing the veriation of the opening degree is not transmitted from a throttle sensor 17 even after a release signal is received from a clutch switch 14 or a brake switch 15, in order to prevent the danger of reckless driving of a car. Then, a signal is sent into an ignitor 18 for controlling the ignition of the engine from the control unit 10. Then, a part or all of the ignition of an ignition coil 19 is cut-off by the ignitor 18, and the number of revolution of the engine is reduced.