A
mass flow rate of
NOx which is recirculated to an intake passage with a blowby gas is obtained with high precision, and based on the result, a state of an
internal combustion engine can be accurately diagnosed. A control device for an
internal combustion engine of the present invention measures a
NOx concentration in an intake passage downstream from a position where the blowby gas is recirculated, and similarly measures an
oxygen concentration in the intake passage downstream from the aforesaid position. Further, the control device measures a
mass flow rate of
fresh air taken into the intake passage. The control device calculates a
mass flow rate of the blowby gas recirculated to the intake passage from the
oxygen concentration and the mass flow rate of the
fresh air. Next, the control device calculates a mass flow rate of all gases in the intake passage from the mass flow rate of the
fresh air and the mass flow rate of the blowby gas. Subsequently, the control device calculates the mass flow rate of
NOx in the aforesaid intake passage from the mass flow rate of all the gases and the NOx concentration. The present control device diagnoses the state of the
internal combustion engine based on the mass flow rate of NOx thus calculated.