An image reading apparatus which is capable of performing accurate shading correction with simple arrangement while preventing a reference member from being scratched and preventing paper powder and the like dust from being deposited on the reference member. The image reading apparatus includes an image reading unit in which a contact glass guides an original on an image reading location. The contact glass is held by a glass holding member. A line image sensor reads the image on the original conveyed on the image reading location, through the contact glass. The image reading unit further includes a reference member whose image is readable by the image sensor, and a moving mechanism to move the image sensor so as to enable the image sensor to alternatively read the original and a reference surface of the reference member.