An internal device of a brain-machine interface system includes: an electrode group including N electrodes, N being 2 or more； an amplification element group including N amplification elements； a communicator communicating with an external device； and a controller selectively executing one of: a normal operation mode in which electroencephalogram signals acquired through the N electrodes 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 N amplified electroencephalogram signals are transmitted； and a noise-reduction operation mode in which an electroencephalogram signal acquired through an M electrode of the electrode group is supplied to the amplification element group in a manner that each M electrode corresponds to respective plural ones of the amplification elements, and an M amplified electroencephalogram signal is transmitted, M being smaller than N.
