A communication system comprising: a host control device; and a vicarious inputting and outputting unit that controls a standard electric device and an extended electric device by communicating with the host control device, wherein: the vicarious inputting and outputting unit comprises a standard board provided with a first control circuit which controls the standard electric device and the extended electric device and at least one of an input circuit and an output circuit for the standard electric device; and 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 at least one specification of an input specification of an input terminal SI and an output specification of an output terminal SO of the extension terminal is set according to the at least one of an input specification of the input circuit and an output specification of the output circuit provided in the extension board through communication with the host control device. a host control device; and a vicarious inputting and outputting unit that controls a standard electric device and an extended electric device by communicating with the host control device, wherein: the vicarious inputting and outputting unit comprises a standard board provided with a first control circuit which controls the standard electric device and the extended electric device and at least one of an input circuit and an output circuit for the standard electric device; and 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 at least one specification of an input specification of an input terminal SI and an output specification of an output terminal SO of the extension terminal is set according to the at least one of an input specification of the input circuit and an output specification of the output circuit provided in the extension board through communication with the host control device. the vicarious inputting and outputting unit comprises a standard board provided with a first control circuit which controls the standard electric device and the extended electric device and at least one of an input circuit and an output circuit for the standard electric device; and 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 at least one specification of an input specification of an input terminal SI and an output specification of an output terminal SO of the extension terminal is set according to the at least one of an input specification of the input circuit and an output specification of the output circuit provided in the extension board through communication with the host control device.