An image reading apparatus which is capable of successfully reading an image off an original by correctly detecting a size of the original without degrading productivity even if mixed-size originals are placed on an original tray with edges not aligned. An image reading unit reads an image off an original passing through a reading position. An original size detection sensor detects an original. A first original size is determined using a result of detection by the original size detection sensor, and a second original size is determined using the image read off the original. The first original size or the second original size is adopted as an original size.