PURPOSE: To diagnose the burning condition of a burner correctly by a method wherein the relative values of an arbitrary number of specified wavelengths among the emission spectrums of two areas of a flame are utilized.

CONSTITUTION: The light of a flame 3 of a burner 2, which is condensed by lenses 4a, 4b, is sent into spectroscopes 6a, 6b from the tip end of optical fibers 5a, 5b through optical fiber cables 7a, 7b, and the light with preselected wavelength is converted into an electric signal, proportional to the strength of the light, to output after amplifying while the electric signal is input to an A/D converter 8 and is sent into a computer 9 after scanning sequentially whereby the behavior of the production, elimination or the like of some specified chemical kinds or the like may be grasped by a formula I through optical analysis in two neighboring areas A, B. With respect to the generation of soot or fine solid particles showing the characteristics of continuous emission, it may be decided whether the generation thereof is progressing or decreasing by analyzing the mutual relations thereof in the same manner and whereby combustion may be decided whether it is good or not.