To provide an engine exhaust gas treatment system that can efficiently reduce and remove NOx using easy-to-handle urea as the reducing agent, regardless of the engine operating conditions. An exhaust gas denitration system in which an exhaust gas is introduced into the denitration catalyst reactor 5 provided in the exhaust gas flue 3, where NOx is reduced and removed in the presence of the denitration catalyst, wherein a diversion means provided in the exhaust gas flue 3 introduces part or all of the exhaust gas into a urea solution injection evaporator having a heating means by the heater 30, and the heat from the exhaust gas and the heat from the heater serve to generate ammonia that is then circled and mixed with the exhaust gas and introduced into the denitration catalyst reactor 4.