An image processing apparatus is provided with a line pair identification unit that includes: a binarization portion that binarizes image data generated by a reading unit reading a document; a detection portion that detects, in an image based on the binarized image data, a line screen pattern in a first direction and a line screen pattern in a second direction that is orthogonal to the first direction; a determination portion that determines the periodicity of the line screen pattern in the first direction; and an identification portion that identifies whether or not a line pair pattern is present in the image data based on the difference in the number between the two line screen patterns as detected by the detection portion and the results of the determination performed by the determination portion.