PURPOSE: To maintain the temperature of a combustion zone at or below a temperature limit set by refractories employed in the combustion zone, by introducing a predetermined volume of sulfur dioxide into the combustion zone in which at least part of H2S is combusted by SO2.

CONSTITUTION: Supply gas containing about 20 to 100% by volume of H2S is introduced by the application of pressure through a conduit 4 into a reaction furnace 2 having a combustion zone. Recirculating fluid stream in a conduit 32 is directly introduced into the reaction furnace 2, and/or introduced into the conduit 4, or a waste heat boiler 8 and/or a heat exchanger 16. The recirculating fluid stream and/or supply gas are controlled so that the volume of SO2 therein introduced into the furnace 2 is about 0.05 k mole to about 0.35 k mole in every one kilomole of H2S in the gas. Thus, any adverse effect on the refractories in the furnace can be prevented.