PURPOSE: To provide an automatically detecting method for the thickness of waste refuse in which no erroneous operation is caused due to a clinker.

CONSTITUTION: An inside part of an incinerator is photographed by the use of a color television camera 2. A color image signal got by the color television camera 2 is separated into three primary color signals of red, green and blue with a three primary color separating circuit. Each of the color signals is made to have multi-values in response to an intensity of each of the colors. The image signals are classified as one of specified numbers of colors in respect to a rate of intensity of each of the three primary colors. It is decided to what color of each of the regions of a specified combustion region 9, a specified noncombustion region 10 and an incinerator wall region 11 does this image signal correspond. A combustion completing level 16 where a color change occurs from the combustion region 9 to the noncombustion region 10 is detected. A first crossing point 13 between this level 16 and the incinerator wall region 11 is detected, a second crossing point 14 where an interface line 14 between a preset fire grate and the incinerator wall is crossed with a vertical line of the first crossing point 13 is detected, a distance ranging from the first crossing point 13 to the second crossing point 14 is calculated and then a thickness 15 at the combustion completing point of waste refuse in contact with the incinerator wall is detected.