An abnormality detection apparatus includes: an air/fuel ratio control portion that controls of fluctuating the air/fuel ratio; a data acquisition portion that acquires a responsiveness parameter during change of output of the sensor between rich and lean peaks; and a abnormality determination portion that when the number of acquisitions reaches or exceeds a first set number, the presence/absence of sensor abnormality based on the average value and the criterion-value is determined, if the average value is not in a pending region; however, if the average value is in the pending region, the data acquisition is performed until the number of acquisitions performed during a large-amount-of-intake-air state of the engine reaches a second set number, and the presence/absence of sensor abnormality based on the average value of the data acquired by the second set number of acquisitions and the criterion-value is determined.