An image reading apparatus used for an image forming apparatus that is coupled therewith and that has a main controller which performs control of the image forming apparatus includes an image reading unit, a detector, and a communication controller. The image reading unit reads an image formed on a document. The detector detects a state of the image reading unit. The communication controller connects to the main controller, the detector, and a communication line. Power is supplied to the detector and the communication controller in both a first operating state, in which supply of power to the main controller is stopped, and a second operating state, in which power is supplied to the main controller. The detector transmits a detection signal to the communication controller in the first operating state, and transmits the detection signal to the main controller in the second operating state.