PURPOSE: To enable detecting a normal operation of an ignition plug and a sure ignition of a burning gas.

CONSTITUTION: Instead of an extermely high secondary voltage of an ignition coil 2, a relatively low primary voltage is detected and after a capacity 11 is charged with the voltage, the capacity 11 is discharged at a certain time constant, thereby producing a charging or discharging waveform voltage. By comparing the voltage with a constant reference voltage, a pulse signal can be obtained. An actuation of the ignition plug 5 may be detected by confirming a presence or an absence of the output of the pulse signal. Another pulse signal is obtained by comparing the charging or discharging waveform voltage which is provided by dividing the primary voltage of the ignition coil 2 with the constant reference voltage vth. If the pulse signal is shaped to have an approximately equal pulse length, a CPU1 may easily process the signal.