An image reading apparatus reads an original-document image and generates image data of the original-document image, and includes an image reading unit, a security pattern detection unit, and a main control unit. The image reading unit reads the original-document image and sequentially generates the image data of the original-document image as a plurality of pieces of band data. The security pattern detection unit performs security pattern detection processing on band data of a single band to detect a portion or an entirety of a security pattern included therein. The main control unit causes the security pattern detection unit to perform the security pattern detection processing on band data of a single band having the lowest background color density and not on band data of a single band having the greatest background color density, among the plurality of pieces of band data.