PURPOSE: To prevent erroneous operation by sensing the room temperature for a predetermined time and determining a temperature variation, comparing it with a stored actual temperature variation rate and determining the degree of fuzzy correlation and indicating abnormality if the degree of fuzzy correlation is abnormal or upon fluctuation of power supply voltage.

CONSTITUTION: A microcomputer 10 is inputted with a power supply voltage VAD 2 from a power supply sensing section 70, a user desired temperature from a key input section 20, and a current temperature from a temperature sensing section 30. The microcomputer 10 performs an inference operation for an inputted temperature according to a specified control regulation and the degree of fuzzy correlation and then makes a decision whether the degree of fuzzy correlation is 80% or above or not. A decision is made that an oil combustor is normal if the degree of fuzzy correlation is 80% or above and then a decision is made whether the power supply voltage VAD 2 sensed at the power supply sensing section 70 is normal or not. If it is normal, a combustion control section 60 sustains the control state. Variation state is presented at a display section 50 if the degree of fuzzy correlation is less than 80% or the power supply voltage is abnormal.