To keep the sulfur oxides concentration in exhaust gas in a regulating value or less, while keeping the furnace outlet temperature in a value for reducing a combustion processing object by increasing the use ratio of inexpensive heavy oil, instead of reducing a use rate of expensive LPG as fuel.

A combustion furnace has a heavy oil burner and an LPG burner, and performs high temperature decomposing processing on plant exhaust gas and/or a plant waste liquid, and is provided with a sulfur oxides concentration measuring instrument 8 measuring the sulfur oxides concentration, and a temperature measuring instrument 4 measuring the exhaust gas temperature of a combustion furnace outlet. A first control circuit controls so as to increase and decrease an LPG flow rate to a quantity of the measured sulfur oxides concentration, and at the same time, a second control circuit controls so as to decrease and increase a heavy oil flow rate to a degree of the measured combustion furnace outlet exhaust gas temperature caused by an increase-decrease in the LPG flow rate.