A signature authentication device is disclosed. Even when there are variations in an input hand-written signature during the registration of the signature or during the authentication of the signature, the signature authentication device easily registers or authenticates the signature. The signature authentication device allows a user to re-register the signature without a risk that sensitive information stored in a computer is monitored by someone else. A plurality of pieces of authentication signature data for a single user is stored. If comparison for authentication between input signature data and at least a single piece of the plurality of the pieces of the authentication signature data is successful, then the authentication is regarded as successful. During signature registration, the plurality of the pieces of the authentication signature data is stored in a storage unit. The signature authentication device determines whether a predetermined startup unit from among a plurality of startup units for starting up the hand-written signature authentication device has started up the signature authentication device, and deletes the signature information and user information when it is determined that the device is started up by the predetermined startup unit.