A communication system includes a host control device and a vicarious inputting and outputting unit that controls an electric device by communicating with the host control device. The vicarious inputting and outputting unit includes a standard board provided with a first control circuit which controls the electric device and at least one of an input circuit and an output circuit for a standard electric device. The first control circuit has an extension terminal for connection to an extension board provided with at least one of an input circuit and an output circuit for an extended electric device and is provided so as to be able to operate in response to respective specifications of plural kinds of extension terminals, and a specification of the extension terminal is set through communication with the host control device.