To increase the commercial value of ash collected by an electrostatic precipitator and to decrease the use amt. of an acidic substance for neutralization by injecting an alkaline neutralizer to a discharged gas to neutralize SO3, recovering SO3 and the unreacted neutralizer in the upstream side of an electrostatic precipitator, and returning to an injecting device of the neutralizer.

A waste gas containing much sulfur oxides such as SO3 produced by combustion of a fuel having a high sulfur content is forcibly exhausted from a boiler 1, and the gas is denitrated in a denitrating device 2. After denitration, the waste gas is at high temp. so that it is sent to a gas air heater 3 to recover the heat. In this course, a neutralizer is injected by a means 15 to neutralize SO3. Then the gas is subjected to heat exchange with combustion air at normal temp. and the combustion air is supplied to the boiler 1. From the mixture of a gypsum powder produced by the neutralization, the unreacted neutralizer powder and ash in the waste gas, the unreacted neutralizer powder is separated by a neutralizer separating means 10, and returned by a supply means 11 after recovery to a neutralizer injecting means 15 for reuse. A part of the recovered neutralizer is used for desulfurization in an absorption tower 7 through a transporting line 12. Thus, by reusing the unreacted neutralizer, the amt. of the neutralizer can be saved.