An image reading apparatus including a controller configured to perform an update processing for updating shading correction data stored in a memory by a generation processing of generating shading correction data based on an image signal of a reference member and perform a size determination processing for determining a size of a document based on an image signal of the document, wherein, when the document on the document platen is read, the controller performs the generation processing before the document is read, and an image processing portion performs the shading correction using the shading correction data generated before the document is read, and wherein, when the size determination processing is performed, the image processing portion performs the shading correction using the shading correction data stored in the memory, and the update processing is not performed.