An image forming apparatus which, when storing image data generated from a read original document in a connected detachable storage device, when the storage cannot be continued, associates the image data which is not stored with authentication information and stored the image data in a storage section included in the image forming apparatus. When a detachable storage device is newly connected, and authentication information is entered, the image forming apparatus determines whether image data associated with the entered authentication information is stored in the image forming apparatus. If it is determined that such image data is stored in the image forming apparatus, the image forming apparatus controls the stored image data to be stored in the newly-connected storage device. On the other hand, if it is not determined that such image data is stored in the image forming apparatus, the image forming apparatus controls the stored image data not to be stored in the newly-connected storage device.