To diagnose a degradation state of an air-fuel ratio sensor based on a behavior at the time when an output of the air-fuel ratio sensor is recovered to its original state after prescribed voltage is impressed between electrodes of the air-fuel ratio sensor on a prescribed engine operating condition for prescribed time.

When a degradation diagnosis condition is formed by an operating state detection means, an output of an air-fuel ratio sensor when the output of the air-fuel ratio sensor is not reversed is stored. After voltage is impressed between electrodes of the air fuel ratio sensor by a voltage impressing means, impression of voltage is stopped, the monitoring of the output of the air fuel ratio sensor is started and the measurement of time elapsed after the voltage impression is stopped is started. Whether the output of the air-fuel ratio sensor after voltage impression is stopped is recovered to a stored output before voltage impression or not is decided. When the output is decided to be recovered, required time till the output is recovered after voltage impression is stopped is compared with a threshold value set based on rotational speed and load of an engine. When required time is decided to be larger than the threshold value, degradation of the air-fuel ratio sensor is diagnosed.