The present invention provides an authentication card inserted to a portable device. The authentication card 1 is inserted (installed) in a cellular phone 30 and the authentication card 2 is inserted (installed) in a PDA 40. The authentication card 1 comprises an identification code transmission unit 10 and the authentication card 2 comprises a usage restriction cancellation unit 20. The units 10 and 20 exchange the identification code by means of wireless signal and mutually authenticate the identification code mutually. In case that the PDA 40 is moved more than the predetermined distance from the cellular phone 30 and the reception level of signal from the unit 10 received by the unit 20 is smaller than the predetermined value, or the unit 20 can not authenticate the identification code of the unit 10, the unit 20 stops transmission of the usage restriction cancellation signal to the PDA 40.