An image reading apparatus which enables reduction of time to be taken from a job start instruction to a reading operation start. An automatic document feeder unit feeds an original. An upstream reader reads an image on one side of the original fed by the automatic document feeder unit. A downstream reader is disposed downstream of the upstream reader in an original feeding direction, to read an image on another side of the original. The upstream reader and the downstream reader read images of respective white reference plates, and the image reading apparatus generates correction data for the upstream reader and the downstream reader based on the images of the respective white reference plates read by the upstream reader and the downstream reader.