It is an object of the present invention to prevent illegal use of a terminal device, and to enhance security of the terminal device itself. Features of the present invention are: to store identification information for collation; to receive identification information for identifying an external communication terminal, which is transmitted from the external communication terminal; to collate the received identification with the stored identification information for collation so as to judge whether or not the received identification information is transmitted from an authorized communication terminal; to set a predetermined function from an execution-disabled state to an execution-enabled state when it is judged by the judgment that the received identification information is transmitted from the authorized communication terminal; and to allow the terminal device to execute the predetermined function.