Provided is an image reading apparatus configured to recognize edge portions of an image of an original precisely without being influenced by the varying state of the four sides of the original. The image reading apparatus includes an image reading unit configured to read the original image by scanning the original, an edge determining unit configured to determine first edge locations at the left end and right end of the original image in a main scanning direction, an edge detector configured to detect a second edge location of the original image in a sub-scanning direction, a tilt expression calculator configured to calculate a linear expression that represents the amount of inclination of the original image with respect to the main scanning direction.