PURPOSE: To reduce the installing area of the title recovering apparatus as well as the manufacturing and construction costs by incorporating a steam generating or superheating part using radiation heat transfer and a steam generating part using convection heat transfer in a combustion furnace in which a sulfur-contg. fluid is burned to manufacture SO2.

CONSTITUTION: Sulfur or a sulfur-contg. fluid such as mercaptan or hydrogen sulfide is spouted from burner 2 and burned in the furnace with dry air introduced from air inlet 3. Boiler feed water is supplied to steam drum 12 from pipe 17, and it enters radiation part heating tubes 4 through descending pipe 13, passes through connection pipe 14 and convection part heating tubes 6, and flows in drum 12. Satd. steam generated is taken out from pipe 16. Otherwise, boiler water from drum 12 is introduced into tubes 6, and steam generated is supplied to the upper parts of tubes 4 from pipe 16 to take out superheated steam from the lower parts of tubes 4.