An image forming apparatus, having an SSD as an auxiliary storage device, which is capable of preventing loss of data in the auxiliary storage device caused by lapse of a retention period in a power save state or a power off state. In a case where the auxiliary storage device is determined to be an SSD based on type information that is acquired when the image forming apparatus is activated, a predicted retention period is calculated, and is set in a part of the apparatus that operates in the power save state. The part of the apparatus that operates in the power save state causes the image forming apparatus to transit from the power save state to a normal state based on the set predicted retention period.