There are provided an image reading apparatus capable of turning on the light source of a display such as an LCD during an image reading operation even when the apparatus operates upon power supply from an external apparatus via a USB bus, and a power control method applied to the apparatus. For example, in the apparatus which is driven upon reception of power supply from the external apparatus, and includes a CIS for reading an original image by intermittently emitting light from an LED, and an LCD with a backlight for displaying information on reading, it is controlled to prevent the ON period of the light source and that of the backlight from overlapping each other during an image reading operation.