PURPOSE: To prevent accidents due to drunken driving by a method wherein signals from an alochol detecting signal discriminting circuit are amplified to operate a relay circuit.

CONSTITUTION: The output of an alcohol sensor 1 in a room is connected to the input of the discriminating circuit 2. The discriminating circuit 2 discriminates the output signal change due to a change of resistance of the alcohol sensor 1 and only when the concentration of alcohol gas exceeds an allowable value the output current flows. A driver circuit 3 amplifies the output current of the discriminating circuit 2, operates a relay and breaks the power source 5 of the starter motor 4 of a car. And, when an output of the discriminating circuit takes 0 the relay does not operate and the power source 5 is not broken.