An exhaust gas purification apparatus of an engine, comprising: a selective reduction catalyst disposed in an exhaust pipe of the engine to selectively reduce a nitrogen oxide by use of ammonia produced from an urea aqueous solution; a dosing unit which doses the urea aqueous solution to exhaust gas on an exhaust upstream side of the selective reduction catalyst; and a control unit having a built-in computer, wherein the control unit calculates an amount of the produced ammonia from an amount of the dosed urea aqueous solution in accordance with an engine operation state, wherein the condition is used that one mol of ammonia is produced from 1⁄2 mol of urea, calculates a purification ratio of the nitrogen oxide by the selective reduction catalyst on the basis of a first nitrogen oxide concentration C U in the exhaust gas discharged from the engine and a second nitrogen oxide concentration C d in the exhaust gas passed through the selective reduction catalyst, wherein the calculated purification ratio equals C u − C d C u, performs calculation to estimate an amount of the ammonia to be adsorbed to the selective reduction catalyst on the basis of the amount of the produced ammonia and the purification ratio of the nitrogen oxide, wherein the estimated amount of ammonia to be adsorbed to the selective reduction catalyst equals the amount of produced ammonia multiplied by 1 − C u − C d C u, corrects the amount of the urea aqueous solution to be dosed in accordance with the engine operation state on the basis of the amount of the adsorbed ammonia, and controls an operation of the dosing unit on the basis of the corrected amount of the urea aqueous solution to be dosed, wherein the control unit further subtracts an amount of the adsorbed urea aqueous solution corresponding to the amount of the adsorbed ammonia from the amount of the urea aqueous solution to be dosed in accordance with the engine operation state, to correct the amount of the urea aqueous solution to be dosed in accordance with the engine operation state. a selective reduction catalyst disposed in an exhaust pipe of the engine to selectively reduce a nitrogen oxide by use of ammonia produced from an urea aqueous solution; a dosing unit which doses the urea aqueous solution to exhaust gas on an exhaust upstream side of the selective reduction catalyst; and a control unit having a built-in computer, wherein the control unit calculates an amount of the produced ammonia from an amount of the dosed urea aqueous solution in accordance with an engine operation state, wherein the condition is used that one mol of ammonia is produced from 1⁄2 mol of urea, calculates a purification ratio of the nitrogen oxide by the selective reduction catalyst on the basis of a first nitrogen oxide concentration C U in the exhaust gas discharged from the engine and a second nitrogen oxide concentration C d in the exhaust gas passed through the selective reduction catalyst, wherein the calculated purification ratio equals C u − C d C u, performs calculation to estimate an amount of the ammonia to be adsorbed to the selective reduction catalyst on the basis of the amount of the produced ammonia and the purification ratio of the nitrogen oxide, wherein the estimated amount of ammonia to be adsorbed to the selective reduction catalyst equals the amount of produced ammonia multiplied by 1 − C u − C d C u, corrects the amount of the urea aqueous solution to be dosed in accordance with the engine operation state on the basis of the amount of the adsorbed ammonia, and controls an operation of the dosing unit on the basis of the corrected amount of the urea aqueous solution to be dosed, wherein the control unit further subtracts an amount of the adsorbed urea aqueous solution corresponding to the amount of the adsorbed ammonia from the amount of the urea aqueous solution to be dosed in accordance with the engine operation state, to correct the amount of the urea aqueous solution to be dosed in accordance with the engine operation state.