An abnormality detection apparatus includes: an air/fuel ratio control portion that performs a control of fluctuating the air/fuel ratio between rich and lean states; a data acquisition portion that acquires, as data for detecting abnormality, a parameter that corresponds to responsiveness during change of output of the A/F sensor between rich and lean peaks during the control; a straight line setting portion that sets a straight line that represents a tendency of change of the parameter relative to change in intake air amount of the engine based on the data acquired by performing acquisition of the data a plurality of times; and an abnormality determination portion that determines the presence/absence of abnormality of the A/F sensor based on comparison between the gradient of the set straight line and an abnormality criterion value.