An image forming apparatus of the present invention comprises a function selection/designation section that obtains the designation of selection of a signed-document creation mode for creating at least signed authentic documents and/or copies thereof, a signature position designation section that obtains the designation of a signature position on printing sheets, a signature input section that obtains signature information input by handwriting, a signature temporary storage section that temporarily stores the signature information from the signature input section, an image obtaining section that obtains image data of an original to be provided with the signature, a signature/image synthesizing section that synthesizes the signature information in the signature temporary storage section and the image data in the image obtaining section so that the signature is provided at the designated signature position on the printing sheet, and a printout section that prints out signed documents based on the data synthesized in the signature/image synthesizing section, according to the signed-document creation mode selected and designated, in such a manner that a designated number of signed authentic documents are discriminated from their copies.