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