An image reading apparatus is provided that includes an image sensor configured to convert image light of a document into an image signal, an optical unit configured to project the image light of the document on the image sensor, a drive unit configured to drive the optical unit in a sub scanning direction, a document size detecting unit configured to detect a size of the document using the image signal, an image signal processing unit configured to digitally convert the image signal of the image sensor into image data, a power circuit configured to output an operating voltage in standby mode and stop the outputting of the operating voltage in sleep mode, and a power saving control unit configured to position the optical unit to a document size detecting position before switching the power circuit from the standby mode to the sleep mode.