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 control circuit unit that measures a background color density of each of the plurality of pieces of the band data; and a main control unit that causes the security pattern detection unit not to perform the security pattern detection processing on the band data of the single band having greatest background color density measured and to perform the security pattern detection processing on the band data of the single band having lowest background color density measured, among the plurality of pieces of the band data, wherein the image reading unit sequentially generates the plurality of pieces of band data for each of a plurality of color planes, and the main control unit causes the security pattern detection unit to perform the security pattern detection processing only for a single color plane having the lowest background color density among the plurality of the color planes, wherein the main control unit identifies a color plane of band data of the single band having the lowest background color density, among the respective pieces of first band data of the plurality of the color planes, as the single color plane having the lowest background color density, wherein the main control unit causes the security pattern detection unit to perform the security pattern detection processing only for a-the single color plane having the lowest background color density among the plurality of the color planes excluding a yellow plane.