An image forming apparatus has a power supply circuit including a power circuit, a main circuit, and a sub-circuit. The sub-circuit includes a timer that performs electrical conduction of a driving contact point of a heater by a first setting time and performs electrical conduction of a driving contact point of a discharge fan by a second setting time when a power switch is turned off. When it is determined that humidity in an image forming apparatus body detected by a humidity detection sensor is equal to or more than a threshold value, the first setting time is set to 0 and the second setting time is set to a predetermined time larger than 0. When it is determined that the humidity is less than the threshold value, the first setting time is set to a predetermined time larger than 0 and the second setting time is set to 0.