An abnormality detection device for an internal combustion engine capable of performing abnormality detection with accuracy, and an air/fuel ratio control apparatus for an internal combustion engine capable of performing air/fuel ratio control with accuracy. An estimated value of the amount of intake air at a valve closing time at which an intake value is closed, is computed. The in-cylinder air/fuel ratio in a cylinder is computed by using the estimated value. The obtained in-cylinder air/fuel ratio is used as an input air/fuel ratio to identify a parameter in a primary delay element. Determination as to the existence/nonexistence of an abnormality in a pre-catalyst sensor (A/F sensor) is made on the basis of the obtained parameter.