An image forming apparatus capable of shifting to a state where quick activation thereof can be performed in a shorter time period when a power switch is turned off. When the off operation of the power switch is detected, a first state is stored, and when the on operation of the power switch is detected next, the state of the image forming apparatus is shifted to a second state from which the image forming apparatus can return to the first state. When the state of the image forming apparatus is shifted to the second state, time starts to be measured. When the on operation of the power switch is detected in the second state, the value of the measured time is acquired. The image forming apparatus is caused to return to the stored first state depending on the value of the measured time.