An image reading apparatus in which the position of the boundary between an image of an original and a background image, as well as a skew and the size of the image of the original, can be accurately detected even with dust on a line image sensor or on an opposing member opposite to the line image sensor. An image formed on an original is read by a reading unit, and an opposing member is arranged at a position opposite to the reading unit. A plate glass distances the original from the opposing member. Shadow portions of the read image is detected from changes in a sub-scanning direction of pixel data of a read image generated based on output from the reading unit, and boundary coordinate data of the image of the original is obtained.