1. An electronic apparatus operable with a detachable battery pack supplying external power and an external response word, the electronic apparatus comprising: a first transmission unit configured to output a request word in response to the external power; a first reception unit configured to receive the external response word; a processing unit; and a display unit to make a display, wherein the processing unit includes an encryption unit configured to encrypt the request word based on a cryptographic algorithm with a common code key; an authentication unit configured to collate the encrypted request word encrypted by the encryption unit and the external response word in order to judge that the battery pack is authenticated when the collated words match and that the battery pack is unauthenticated when the collated words do not match; and an operation restriction unit, wherein, when the authentication unit judges that the battery pack is unauthenticated, the operation restriction unit restricts a part of an operation of the electronic apparatus in order to prohibit a Web function but permit a telephone call function, and the display unit makes a display to indicate that a part of the operation is restricted, wherein the Web function includes Internet and e-mail functions.