Provided is an image forming apparatus control method which, when an image is to be formed from image data with a digital signature, safely manages the image formed from the image data even if it is impossible to certify whether the image data is tampered. When receiving a request for formation of an image of image data with a digital signature from a personal computer, this image input/output apparatus requests a digital signature issue/authentication server to certify the authenticity of the signature data. On the basis of the certification result, the apparatus adds, to the output image, an additional image indicating that the original image is tampered, or an additional image indicating that whether the original image is tampered cannot be certified.