An exhaust gas purifying apparatus of an engine comprising: a selective reduction catalyst provided in an exhaust passage of the engine; an ammonia precursor supply unit that supplies an ammonia precursor to an upstream side of the selective reduction catalyst and breaks down the ammonia precursor in the exhaust passage to cause the generated ammonia to flow into the selective reduction catalyst, thereby to allow the selective reduction catalyst to reduce and purify nitrogen oxides in exhaust gas of the engine; an exhaust gas recirculation passage that introduces the exhaust gas from the exhaust passage and recirculates the exhaust gas to an intake passage of the internal combustion engine; an exhaust gas recirculation rate control unit that controls a flow rate of the exhaust gas recirculated into the intake passage through the exhaust gas recirculation passage; and the exhaust gas purifying apparatus of the engine characterized by further comprising: a control unit that sets a ratio between a supply rate of the ammonia precursor and a recirculation rate of the exhaust gas set based on an operating state of the engine, and, based on the ratio, controls the operation of the ammonia precursor supply unit and the exhaust gas recirculation rate control unit, an exhaust gas temperature detection unit that detects an exhaust gas temperature in the exhaust passage between the ammonia precursor supply unit and the selective reduction catalyst; and a change unit that changes the ratio between the supply rate of the ammonia precursor and the recirculation rate of the exhaust gas set by the control unit so that a reduction in the exhaust gas temperature detected by the exhaust gas temperature detection unit reduces the supply rate of the ammonia precursor and increases the recirculation rate of the exhaust gas, wherein the control unit places the supply of the ammonia precursor in a wait state until the exhaust gas temperature is greater than or equal to a predetermined temperature allowing the ammonia precursor to be hydrolyzed, and causes the exhaust gas recirculation rate control unit to increase the flow rate of the exhaust gas to be recirculated into the intake passage while in the wait state. a selective reduction catalyst provided in an exhaust passage of the engine; an ammonia precursor supply unit that supplies an ammonia precursor to an upstream side of the selective reduction catalyst and breaks down the ammonia precursor in the exhaust passage to cause the generated ammonia to flow into the selective reduction catalyst, thereby to allow the selective reduction catalyst to reduce and purify nitrogen oxides in exhaust gas of the engine; an exhaust gas recirculation passage that introduces the exhaust gas from the exhaust passage and recirculates the exhaust gas to an intake passage of the internal combustion engine; an exhaust gas recirculation rate control unit that controls a flow rate of the exhaust gas recirculated into the intake passage through the exhaust gas recirculation passage; and the exhaust gas purifying apparatus of the engine characterized by further comprising: a control unit that sets a ratio between a supply rate of the ammonia precursor and a recirculation rate of the exhaust gas set based on an operating state of the engine, and, based on the ratio, controls the operation of the ammonia precursor supply unit and the exhaust gas recirculation rate control unit, an exhaust gas temperature detection unit that detects an exhaust gas temperature in the exhaust passage between the ammonia precursor supply unit and the selective reduction catalyst; and a change unit that changes the ratio between the supply rate of the ammonia precursor and the recirculation rate of the exhaust gas set by the control unit so that a reduction in the exhaust gas temperature detected by the exhaust gas temperature detection unit reduces the supply rate of the ammonia precursor and increases the recirculation rate of the exhaust gas, wherein the control unit places the supply of the ammonia precursor in a wait state until the exhaust gas temperature is greater than or equal to a predetermined temperature allowing the ammonia precursor to be hydrolyzed, and causes the exhaust gas recirculation rate control unit to increase the flow rate of the exhaust gas to be recirculated into the intake passage while in the wait state. a control unit that sets a ratio between a supply rate of the ammonia precursor and a recirculation rate of the exhaust gas set based on an operating state of the engine, and, based on the ratio, controls the operation of the ammonia precursor supply unit and the exhaust gas recirculation rate control unit, an exhaust gas temperature detection unit that detects an exhaust gas temperature in the exhaust passage between the ammonia precursor supply unit and the selective reduction catalyst; and a change unit that changes the ratio between the supply rate of the ammonia precursor and the recirculation rate of the exhaust gas set by the control unit so that a reduction in the exhaust gas temperature detected by the exhaust gas temperature detection unit reduces the supply rate of the ammonia precursor and increases the recirculation rate of the exhaust gas, wherein the control unit places the supply of the ammonia precursor in a wait state until the exhaust gas temperature is greater than or equal to a predetermined temperature allowing the ammonia precursor to be hydrolyzed, and causes the exhaust gas recirculation rate control unit to increase the flow rate of the exhaust gas to be recirculated into the intake passage while in the wait state.