A control device of an engine comprising: means for supplying a fuel to a combustion chamber; means for setting an amount of the fuel to be supplied to the combustion chamber by the fuel supplying means to a target fuel supply amount; means for calculating on the basis of the target fuel supply amount a fuel supply command value for making the fuel supplying means supply the fuel of the target fuel supply amount set by the target fuel supply amount setting means to the combustion chamber and providing the fuel supply command value to the fuel supplying means; means for detecting an amount of an air supplied to the combustion chamber; means for detecting an air-fuel ratio of a mixture gas formed in the combustion chamber; and means for calculating an air-fuel ratio of the mixture gas formed in the combustion chamber on the basis of a fuel supply amount indicated by the fuel supply command value and the amount of the air detected by the supplied air amount detection means; characterized by further comprising: means for detecting a concentration of a specified component included in an exhaust gas discharged from the combustion chamber; and means for calculating the concentration of the specified component included in the exhaust gas discharged from the combustion chamber on the basis of the fuel supply amount indicated by the fuel supply command value and the amount of the air detected by the supplied air amount detection means, wherein the fuel supply command value or the fuel supply amount indicated thereby is amended and the amount of the air detected by the supplied air amount detection means is amended such that the difference between the air-fuel ratio detected by the air-fuel ratio detection means and the air-fuel ratio calculated by the air-fuel ratio calculation means and the difference between the concentration of the specified component detected by the exhaust gas component concentration detection means and the concentration of the specified component calculated by the exhaust gas component concentration calculation means become small, wherein the specified component is a nitrogen oxide included in the exhaust gas. means for supplying a fuel to a combustion chamber; means for setting an amount of the fuel to be supplied to the combustion chamber by the fuel supplying means to a target fuel supply amount; means for calculating on the basis of the target fuel supply amount a fuel supply command value for making the fuel supplying means supply the fuel of the target fuel supply amount set by the target fuel supply amount setting means to the combustion chamber and providing the fuel supply command value to the fuel supplying means; means for detecting an amount of an air supplied to the combustion chamber; means for detecting an air-fuel ratio of a mixture gas formed in the combustion chamber; and means for calculating an air-fuel ratio of the mixture gas formed in the combustion chamber on the basis of a fuel supply amount indicated by the fuel supply command value and the amount of the air detected by the supplied air amount detection means; means for detecting a concentration of a specified component included in an exhaust gas discharged from the combustion chamber; and means for calculating the concentration of the specified component included in the exhaust gas discharged from the combustion chamber on the basis of the fuel supply amount indicated by the fuel supply command value and the amount of the air detected by the supplied air amount detection means, wherein the fuel supply command value or the fuel supply amount indicated thereby is amended and the amount of the air detected by the supplied air amount detection means is amended such that the difference between the air-fuel ratio detected by the air-fuel ratio detection means and the air-fuel ratio calculated by the air-fuel ratio calculation means and the difference between the concentration of the specified component detected by the exhaust gas component concentration detection means and the concentration of the specified component calculated by the exhaust gas component concentration calculation means become small, wherein the specified component is a nitrogen oxide included in the exhaust gas.