An abnormality diagnosis device according to the present invention can calculate a calculation air-fuel ratio A/Fk based upon a fuel injection quantity and an intake air quantity, and determine the abnormality of the air-fuel ratio sensor based upon the calculation air-fuel ratio and an actual air-fuel ratio detected by an air-fuel ratio sensor, when an operating condition in the internal combustion engine relatively rapidly changes according to an engine operating demand and thereby the calculation air-fuel ratio relatively rapidly changes. Since the air-fuel ratio is not forcibly changed and fluctuated, deterioration of the exhaust emission and the combustion state, and deterioration of the vibration and the noise at abnormality diagnosing can be certainly prevented.