In an exhaust gas treatment device provided with: an exhaust gas treatment unit in which an oxygen combustion boiler 1 using coal as fuel, a denitration device 3, an air preheater 4, a dust-collection device 5, a desulfurization device 6, and a carbon dioxide recovery device 8 are sequentially arranged from the upstream side to the downstream side of an exhaust gas duct; and an exhaust gas circulation unit which branches off from the exhaust gas duct at an outlet of the dust-collection device 5 or an outlet of the desulfurization device 6 and through which the exhaust gas is preheated by the air preheater 4 and returned to the oxygen combustion boiler 1, a heat-recovery heat exchanger 13 that adjusts a gas temperature at an inlet of the dust-collection device 5 to be not greater than an acid dew point of SO3 and not lower than a water dew point is provided between the air preheater 4 and the dust-collection device 5, a reheating heat exchanger 13 that adjusts a gas temperature to be not lower than the acid dew point of SO3 is provided in the vicinity of a branch of the exhaust gas circulation unit, adjusting the gas temperature at the inlet of the dust-collection device 4 to be not greater than the acid dew point of SO3 makes it possible to prevent corrosion of pipes in the exhaust gas circulation unit and to avoid a decrease in fluidity or combustibility of the pulverized coal in the pipes in the mill.