PURPOSE: To enhance reliability of a detecting circuit for a pilot flame, by forming a detecting circuit which can detect an insulation resistance between a pilot flame producer and a flame rod, as well as detecting the existence of a pilot flame in the form of a flame current.

CONSTITUTION: The output from a comparator 19 is in high level when a pilot flame does not exist in a pilot flame producer 4 and a flame current is not generated. But when a pilot flame exists and a flame current is generated, the voltage is produced in the capacitor 18 of an integrated circuit, and an output from a capacitor 19 is reduced to low level. On the other hand, when the connecbypassed by a diode 24. An output from a comparator 28 turns to low level, when the degrees of insulation is lowered by soot which is produced from pilot flame. As a result, insulating condition whether it is good or bad between a pilot flame producer 4 and a flame rod 6 can be detected.