An internal device (1) is implanted in a living body (300). A control section (6) causes a communication section (5) to wirelessly transmit data corresponding to electroencephalogram signals of the living body (300) which are detected through a group of N (N is 2 or more) electrodes, to an external device (200). When the communication section (5) receives a designation signal designating a group of M electrode(s) (2a), M being smaller than N, the communication section (5) is caused to transmit data corresponding to electroencephalogram signals of the living body (300) which are detected through the group of M electrode(s) (2a), to the external device (200) in real time.
