PURPOSE: To stop an engine instantly by operating a control transistor between the base and emitter of a main transistor by the charge voltage signal of a main condenser and the electric potential of a condender in which a series circuit consisting of an engine stop switch and a condenser is connected at both the edges of a battery.

CONSTITUTION: When the charge voltage of a condenser C0 becomes over a prescribed value during the engine operation, also the voltage of a condenser C3 becomes high, and a control transistor Q2 is turned ON through a resistor R4 diode D4, and a constant voltage diode Z1, and the base electric current of the transistor Q1 is bypassed, and the overvoltage of output is prevented. On stop, a switch SW2 is turned ON, and then a condenser C1 is charged by the voltage of a battery, and at the same time, the base of the control transistor Q2 is applied with the voltage through a diode D8, and turned ON. Accordingly, the base electric current of the main transistor Q1 is cut off, and the oscillation of a converter is suspended, and the electric charge of the condenser Q0 is reduced to zero.