PURPOSE: To make it possible with no man attending a furnace to know exactly where the most active combustion is located and judge the state of combustion in the furnace by providing two video cameras, a device to judge in the video picture a section where the luminance is most high out of the luminance distribution in the video picture, and a device to calculate three dimensional positions.

CONSTITUTION: Pictures to show the state of combustion of wastes in a furnace 1 are taken by video cameras 2 and 3, and they are sent as image signals to a monitor device 4 and a digitizer 5. The video cameras 2 and 3 are provided as needed with a dust-proof mechanism, a cooling mechanism, etc. The digitizer 5 divides and digitize into a plurality of stages of the luminance the image signals from the video cameras 2 and 3 and those stages are digitized. Output signals of the digitizer 5 which have been digitized are sent to an arithmetic unit 6, and on a photo image picture n-many frames of a certain size are set up laterally to cover the picture, and for each frame areas of individual luminances are repeatedly calculated. With this arrangement the state of combustion in the furnace can be judged unattended, and the accuracy of the judgement can be much improved.