In an exhaust gas treatment system including a denitration device that removes nitrogen oxide in exhaust gas from a heavy fuel-fired boiler, an air preheater that recovers heat in the gas after the nitrogen oxide is removed, an electric precipitator that removes dust while adding ammonia into the gas after heat recovery, a desulfurization device that removes sulfur oxide in the gas after dust removal, and a stack that exhausts the gas after desulfurization to the outside, an ash-shear-force measuring instrument is provided to measure an ash shear force, which is ash flowability, on the downstream side of the electric precipitator, so that a feed rate of an air supply unit that supplies air to the boiler is reduced according to ash shear-force information.