A user's handwritten signature data is transmitted to a host device, when the user registers his or her signature. A signature information controlling unit reads the codes of a character string of a font closest to handwriting shape data by referencing a signature dictionary information unit, extracts a predetermined number of digits of numeric values from the codes, generates a user ID by making the numeric values successive to the number of codes, and registers this user ID and the input handwritten signature to the signature dictionary information unit by making a correspondence between them. When a signature is authenticated, a user ID is automatically generated from a handwritten signature input by a user, and checked between generated user ID and registered user ID.