An image forming apparatus includes a main power supply section, an auxiliary power supply section, a plurality of sensors, and a power control section. The sensors are disposed at a plurality of positions. The sensors change output levels thereof upon detection of a user's operation for initiating an image forming process. The power control section switches the apparatus from a normal operation mode to the power-saving operation mode after a predetermined time period during which no image forming process is performed. The power control section detects the respective output levels of the sensors through sequential power supply from the auxiliary power supply section to the sensors. The power control section switches the apparatus to the normal operation mode upon detection of a change in output level of either one of the sensors.