An image forming apparatus includes an image controller that performs processing of input image information, an engine controller into which a first processing part and a second processing part are combined, and a power supply that supplies power to the engine controller. A mode of the image forming apparatus is switchable from a first mode that is a state of image formation or a stand-by state, to a second mode whose power consumption is smaller than a power consumption of the first mode. In addition, a switch unit switches between the first mode in which power is supplied to the second processing part from the power supply. The image controller and the first processing part communicate with each other through the second processing part in the first mode, or the image controller and the first processing part communicate with each other through a communication line but not through the second processing part in the second mode. In the second mode the first processing part controls the switch unit to start supplying power to the second processing part when the image controller orders transition from the second mode to the first mode through the communication line.