An adapter in which a portable telephone having a socket including terminals connected to electric terminals therein is receivable includes an adapter body having a concave portion in which the portable telephone is receivable, a connector provided in the adapter body so as to be exposed to the concave portion, the connector having terminals connected to predetermined electric units including at least an antenna, the connector being connectable the socket of the portable telephone, and a clamping mechanism for clamping the portable telephone received in the concave portion of the adapter body in a state where the connector is connected to the socket of the portable telephone so that the terminals of the connector are in contact with the terminals of the socket of the portable telephone.