According to an embodiment, an image forming apparatus includes: a detecting unit, a storing unit, a threshold determining unit, and a control unit. The detecting unit detects an ambient brightness level at an arbitrary time zone. The storing unit stores information indicating the brightness level detected by the detecting unit. The threshold determining unit determines a threshold serving as a criterion for switching into a low-power-consumption mode according to the information indicating the brightness level stored in the storing unit. The control unit controls switching into the low-power-consumption mode using the threshold determined by the threshold determining unit.