1. An image processing apparatus comprising: a reader that reads a document and generates image data; a detector that detects a line screen pattern in a first direction in an image based on the image data generated by the reader and a line screen pattern in a second direction that is orthogonal to the first direction; an identifier that identifies whether or not a line pair pattern is present in the image data based on the difference in the number of the two line screen patterns detected by the detector; a binarization portion that binarizes the image data; wherein the detector detects the number of transition points between ON-pixels and OFF-pixels in the first direction and the number of transition points between ON-pixels and OFF-pixels in the second direction of an image based on the image data binarized by the binarization portion; and the identifier identifies whether or not a line pair pattern is present in the image data based on the difference in the number of transition points between ON-pixels and OFF-pixels in the first direction and the number of transition points between ON-pixels and OFF-pixels in the second direction.