There is provided a dither control means that performs a dither control, which compulsively changes an air-fuel ratio alternately to rich side and to lean side by increasing and decreasing fuel injection quantity of an injector (fuel injection valve) in a stepped manner. A predicted value (ideal A/F value ID), which indicates an ideal change of a detection value of an A/F sensor (oxygen concentration sensor) in a case where the A/F sensor is not degraded, is set as a standard value. Then, an integral of a difference between the detection value of the A/F sensor, which changes in accordance with the dither control, and the standard value is calculated. If a calculated value of the integral is larger than a predetermined value, it is determined that the A/F sensor is degraded.