PURPOSE: To prevent engine stop and permit self traveling by permitting switching to the single operation of capacitive discharge or electric-current cut-off by a switching element, when the ON/OFF state of a power transistor is observed and the trouble of said transistor is detected.

CONSTITUTION: When a power transistor Tr15 controlled by an ignition-signal generating circuit 14 shifts to ON state from OFF state, the pulses having a certain width are output from a single multi 21 in an anomaly detecting circuit 34, and a condenser 24 is charged by the voltage which increases with the increase of the engine speed. In this case, a voltage comparator 29 always outputs the voltage at H level if the transistor Tr15 operates normally. When the transistor Tr15 is broken and short-circuit state is formed, an output is not generated from the single multi 21, and the charged voltage of the condenser 24 becomes zero, and the output of the voltage comparator 29 becomes the signal at L level. Then, a relay 31 is turned ON, and relay contacts 31a and 31b are switched to the normal open side, and an ignition coil 3 is directly connected and disconnected by an interrupter 4.