PURPOSE: To treat exhaust gas according to the concn. of SO2 by installing an auxiliary heat exchanger in a gas passage connecting a first-stage catalyst bed to a first-stage heat exchanger.

CONSTITUTION: An auxiliary heat exchanger 5 is installed between a first-stage catalyst bed in a converter 2 and a first-stage heat exchanger 3 so as to reduce the temp. of gas flowing in the first-stage heat exchanger 3. Air for reducing the temp. of the gas leaving the first-stage catalyst bed in the converter 2 and flowing in the first-stage heat exchanger 3 is introduced into the auxiliary heat exchanger 5 from the outside except a sulfuric acid plant through a blower 6 for a smelting furnace 1. A bypass 8 detouring the auxiliary heat exchanger 5 is joined to a passage 7 for introducing the external air into the heat exchanger 5 and the degree of opening of a valve 9a is regulated to control the amt. of the external air introduced into the heat exchanger 5. A high rate of conversion is maintained in the first-stage catalyst bed and energy saving effect is enhanced.