PURPOSE: To prevent a distributor from being overheated and damaged by limiting the operating period of a auxiliary power source for a ignition device of an internal-combustion engine only to desired timing and further limitting operating timing of the auxiliary power source to a necessary spark discharge period.

CONSTITUTION: When a main switch 4 is turned on to start an engine, discharge takes place in a gap 8. The discharge current flows through impedance R1 and voltage across both ends of the impedance is differentiated by a differential circuit 10 so that the output of discharge detecting pulses P0, P1, P2... are generated as differential outputs. In Ti.(α+β) time after the generation of pulse P1, a timer 16 is counted up to produce the output for setting a flip-flop 17 the Q output of which becomes 1 to operate a DC-DC converter 6 and reinforce the spark discharge. A timer 15 is preset to Ti.α by pulse P2 and then generates the output to reset the flip-flop 17. Thus the Q output becomes 0 to stop the operation of the converter 6.