PURPOSE: To obtain stable and efficient combustion always by keeping the superficial linear velocity in a column automatically at an optimal value.

CONSTITUTION: Minimal conditions to be observed by an incinerator for treating industrial waste are satisfied by using the amount V of fluidizing air supplied into the furnace as control factor, and controlling the amount of fluidizing air such that incomplete combustion is caused in fluidized bed, by starting operation of the furnace with a minimal amount of fluidizing air V0, checking always the density of residual O2 in exhaust gas during operation of the furnace, controlling the amount of fluidizing air to V1 (V0>V1) capable of keeping an optimal superficial linear velocity in a column in case that the density of residual O2 is higher than that determined by the percentage of excess air for keeping complete combustion in the furnace, and controlling the amount of fluidizing air to V0 in case that the density of residual O2 is lower than that determined by the percentage of excess air for keeping complete combustion in the furnace.