PURPOSE: To discriminate an ion current value during an abnormal lighting time from that during a normal lighting time and judge the combustion state to be normal/abnormal by judging the combustion state by ion current in flames and flashing the indication of the ion current value when the combustion state is judged to be abnormal.

CONSTITUTION: It is judged if a combustion switch is throw in. When the switch is turned on already, an ion current value is measured and stored. Comparison is made between the factual ion current value and the preset reference ion current value so as to decide if the combustion is normal. When a burner produces imperfect combustion, the ion current value tends to decrease, which makes it possible to judge whether the combustion is normal or abnormal. When it is judged to be normal combustion, the ion current value is displayed at a digital value on a display 11. When it is judged to be abnormal combustion, the digital value is arranged to flash. Therefore, a user is capable of knowing abnormal combustion at a glance at the indicator 11.