PURPOSE: To diagnose the combustion state by a method wherein flame configurations are detected by image flbers so as to determine the flame pattern in order to compare with the reference flame pattern memorized in advance.

CONSTITUTION: Image signals at the roots of the respective flames 1 detected by a detecting part are converted at an image pickup device 6 to analog signals and then converted again at an A/D converter 8 to digital signals in order to be brought into an electronic computer 9. The image signals brought into the computer in the form of digital signals are compared with the reference flame pattern memorized in advance. In such a manner as described above, the combstion state of a boiler can be grasped automatically, promptly and surely.