PURPOSE: To enable self-inspection of simple structure, long lifetime, and high reliability by comprising a checking photodiode, a light source, an operation means, a judgment circuit, etc.

CONSTITUTION: In an operational amplifier, a photodiode(PD) 3 of a sensor for detecting the state of flame 2 is connected to the non-inversion terminal, and a checking PD 4 is connected reversely in series to the PD 3 and optically to the light source 5, which is on-off controlled periodically by a drive circuit 6. A judgment circuit 7 which inputs output voltage e0 to judge the state of flame and the state of each part connected to the fore stage is connected to the output terminal of the operational amplifier, and a feedback resistor 8 of resistance value Rr between the output terminal and the non-inversion terminal. The circuit 7 judges that each part of the fore stage is normal only when input voltage e0 is a pulse output of fixed period, reverses the polarity of voltage e0 by a polarity inversion circuit, and judges that a flame state is normal only when a signal from this circuit is a pulse output of fixed period.