An image forming apparatus capable of more quickly executing a fail-safe function for shifting to a power state different in power consumption. The apparatus can operate in power states different in power consumption. A controller controls the apparatus. A power supply controller controls supply of power. A reset circuit resets the controller. When a power switch is operated by a user, a monitoring time period, set in advance, is decided according to a type of user's operation and the power state. One of operations to be executed when the monitoring time period has elapsed, for causing the controller to cut off the supply of power to the controller and for causing the reset circuit to reset the controller, is set according to the type of user's operation and the power state. The set operation is executed when the decided monitoring time period has elapsed.