To facilitate the measurement of limit of sludge mixing rate with respect to waste solidified fuel and the incinerating treatment of sludge, by a method wherein a fluidized bed is formed in an incinerating chamber installed in a heating furnace by a gas supplying means, and combustion gas in the incinerating chamber is induced through a sampling means while the constituent of the combustion gas is analyzed by an analyzing means.

Waste to be incinerated is filled into a quartz pipe 1 (incinerating chamber 2) in a heating furnace A while gas, whose amount of oxygen is regulated, is blown into the incinerating chamber 2 from a gas supplying means B to form a fluidized bed 4 on a dispersing plate 3 and incinerate the matter to be incinerated. Combustion gas in the incinerating chamber 2 is sampled by a sampling means C while the sample is analyzed by an analyzing means D to investigate the nature of various kinds of gas contained in the combustion gas. Thereafter, data are compared with reference values in a data treating means 10 based on the kind and concentration of harmful matters to decide the mixing rate of materials forming the matter to be incinerated. According to this method, the limit of mixing range of sludge is measured and the sludge is incinerated easily.