PURPOSE: To enable suppressing an increase in the concentration of SO3 accompanying elevation in temperature of the exhaust gas from the combustion and an increase in the proportion of SO3, in the exhaust gas passed though an air preheater so that corrosion of the equipment, such as a gas heater, located on the downstream side of an electrostatic precipitator can be prevented.

CONSTITUTION: An outside air-drawing line 14 with an intake 13 opening outside a boiler chamber 2 is connected at a suitable point on the upstream side of a forced draft fan 5 in a combustion air-feeding line 3. An outside air-drawing damper 15 is provided at an intermediate point in the outside air-drawing line 14. In the combustion air-feeding line 3 a boiler chamber air-drawing damper 16 is provided at an intermediate point on the upstream side of the connection point of the outside air-drawing line 14.