PURPOSE: To prevent defective operation and electric shock by a construction wherein a semiconductor switching element is provided between an output end of a generating coil and a stop switch, and the control electrode is connected to a counter output side of the switch through a constant voltage means.

CONSTITUTION: A semiconductor switching element 13 is provided between an output end of a generating coil 1 and a stop switch 10, and a constant voltage means is provided between the control electrode and a terminal on counter output side of the switch 10. For example, diodes 15, 16 connected in series are employed for the constant voltage means, which are connected to a thyristor 13, a resistance 11 and a diode 14 as illustrated, thus constituting a shut-down circuit 17. From closing the switch 10 for shutting down an engine, a drop voltage about 1.4V of the diodes 15, 16 is impressed between gate and cathode of the thyristor 13, which becomes conducting in consequence, an output of the generating coil 1 is short-circuited by the thyristor 13 and the switch 10, a condenser 4 is not charged, and thus the engine is shut down securely.