A device for controlling an internal combustion engine capable of estimating the amount of NOx emission within short periods of time maintaining high precision and realizing improved control performance without increasing the cost as a result of not using map data in the ROM. The device includes NOx operation circuit for estimating the amount of NOx in the exhaust gas from a theoretical formula and an empirical formula based upon the intake air amount Qa, intake air temperature To, pressure Pb, air-fuel ratio &lgr; and EGR rate &bgr;, and control circuit for controlling at least either the NOx purifying catalyst or the combustion state in the internal combustion engine in order to lower the amount of NOx emission.