PURPOSE: To obtain a flame detection circuit free of occurrence of hazardous state, e.g., fire, etc., by using a system in which alarm is always sento out even when a flame eye becomes out of order.

CONSTITUTION: By using an arithmetic amplifier 30 of the same input voltage characteristics with transfer characteristic as a comparator in which signal from a flame eye 12 is compared with reference voltage and by the output an ignition control relay 13 is ON-OFF controlled, when the signal from the flame eye is below the transfer level of the arithmetic amplifier 30, output similar to the dark state of the flame eye is taken out of the arithmetic amplifier to put the ignition control relay to ON-operation. Thus, since alarm operation treatment can be finally made in the combustion controller even when the flame eye is broken down by short-circuit, a flame detection circuit having a very high safety, without defects, e.g., nonstop release of fuel, etc., can be obtained.