PURPOSE: To provide a combustion diagnostic device capable of constituting a low-cost circuit which can detect the quantity of light with a good accuracy against a change in atmospheric temperature and eliminate the necessity of regulating the circuit.

CONSTITUTION: A temperature sensor 19, detecting an atmospheric temperature in an atmosphere wherein a photodetector 10 (photodiode) and an operational amplifier are installed, is mounted to monitor a temperature by a microcomputer 13 at all times to subtract the element of error, depending on a temperature, from the output of a signal amplifier 11 through a software whereby the element of error can be removed with a good accuracy employing the comparatively inexpensive operational amplifier.