To provide an exhaust gas treatment method capable of reducing the amount of SO3 flowing out on the downstream side by stably and efficiently collecting ammonium sulfate by an electric precipitator in a process for injecting ammonia to react the same with SO3 in exhaust gas to stably form ammonium sulfate and capable of preventing trouble such as the adhesion of ammonium sulfate to a reheater on a heat recovery side generated in a post process.

A fine particulate solid substance SD such as collected dust or the like is injected on the upstream side of a position where ammonia AN is injected in the exhaust gas pipe between an air heater 3 and the electric precipitator 4 and allowed to function as a nucleus when SO3 in exhaust gas is reacted with ammonia A injected in the exhaust gas pipe and ammonium sulfate is precipitated by vapor phase precipitation reaction and the fine particulate solid substance SD precipitated on the surface of ammonium sulfate is stably and efficiently collected along with combustion soot contained in the exhaust gas by the electric precipitator 4.