PURPOSE: To generally diagnose a combustion state in a boiler furnace in which two-stage combustions are conducted.

CONSTITUTION: Lights guided from optical probes 4, 18, 19 are guided to a spectroscope 7 through a switching unit 22, spectrally analyzed to luminances of respective wavelengths by the spectroscope 7, then converted to electric signals 9 by a photoelectric converter 8, input to a calculator 10, calculated by the calculator 10, and combustion states of a burner combustion zone 12, a two-stage combustion zone 15 and a furnace outlet 16 are diagnosed. Thus, in addition to a diagnosis of the combustion state in a conventional burner combustion zone 12, the combustion states of the zone 15 for affecting large influence to combustion behavior in a boiler furnace 2 and the outlet 16 can be quantitatively evaluated and diagnosed according to a temperature distribution in a lateral direction of the furnace, a temperature variation, etc.