In an image-reading apparatus, the size of a document placed at a predetermined position is sensed by a document size sensor, an image is read from the document in the document size sensed, and image data indicative of the read image is subjected to certain processing executed by an image processing section. A power supply section is provided which includes a first power supply which supplies power to the image processing section, and a second power supply which supplies power to the document size sensor. The second power supply is supplied with power from the first power supply and can be turned on and off independently of turn-on and turn-off of the first power supply. A power supply control section is also provided which controls turn-on and turn-off of the second power supply.