1. An image forming apparatus comprising: an input unit configured to input image data; an image forming unit configured to form an image on a recording material based on the image data input by the input unit; a reading unit configured to read the image formed on the recording material by the image forming unit; and a control unit configured to, based on the input image data and the image read by the reading unit, acquire a plurality of change amounts from the read image, and determine a type of an abnormal image based on a plurality of the acquired change amounts.