An internal device of a brain-machine interface system, the internal device adapted to be implanted in a living body, the internal device comprising: an electrode group which includes N electrodes acquiring electroencephalogram signals of the living body, wherein N is 2 or more; an amplification element group which includes N amplification elements; and a communicator which is configured to wirelessly communicate with an external device; characterized by a controller which is configured to control an electrical connection between the electrode group and the amplification element group, and which is configured to control the communicator, the controller which is configured to selectively execute one of: a normal operation mode in which electroencephalogram signals of the living body that are acquired through the N electrodes of the electrode group are supplied to the amplification element group in a manner that each of the N electrodes corresponds to a respective one of the N amplification elements, and the communicator is caused to wirelessly transmit N amplified electroencephalogram signals; and a noise-reduction operation mode in which an electroencephalogram signal of the living body that is acquired through M electrodes selected from the N electrodes of the electrode group is supplied to the amplification element group in a manner that each of the M electrodes corresponds to respective plural ones of the amplification elements, and the communicator is caused to wirelessly transmit M amplified electroencephalogram signals, wherein M is smaller than N. an electrode group which includes N electrodes acquiring electroencephalogram signals of the living body, wherein N is 2 or more; an amplification element group which includes N amplification elements; and a communicator which is configured to wirelessly communicate with an external device; characterized by a controller which is configured to control an electrical connection between the electrode group and the amplification element group, and which is configured to control the communicator, the controller which is configured to selectively execute one of: a normal operation mode in which electroencephalogram signals of the living body that are acquired through the N electrodes of the electrode group are supplied to the amplification element group in a manner that each of the N electrodes corresponds to a respective one of the N amplification elements, and the communicator is caused to wirelessly transmit N amplified electroencephalogram signals; and a noise-reduction operation mode in which an electroencephalogram signal of the living body that is acquired through M electrodes selected from the N electrodes of the electrode group is supplied to the amplification element group in a manner that each of the M electrodes corresponds to respective plural ones of the amplification elements, and the communicator is caused to wirelessly transmit M amplified electroencephalogram signals, wherein M is smaller than N. a normal operation mode in which electroencephalogram signals of the living body that are acquired through the N electrodes of the electrode group are supplied to the amplification element group in a manner that each of the N electrodes corresponds to a respective one of the N amplification elements, and the communicator is caused to wirelessly transmit N amplified electroencephalogram signals; and a noise-reduction operation mode in which an electroencephalogram signal of the living body that is acquired through M electrodes selected from the N electrodes of the electrode group is supplied to the amplification element group in a manner that each of the M electrodes corresponds to respective plural ones of the amplification elements, and the communicator is caused to wirelessly transmit M amplified electroencephalogram signals, wherein M is smaller than N.