Disclosed is a multiplex communication system which comprises a master unit, and a plurality of slave units, wherein the master unit and the plurality of slave units are annularly connected by a signal line. The master unit transmits a request signal to each of the slave units via the signal line and receives a response signal in response to the request signal from each of the slave unit. An area for which the plurality of slave units can write data is set for the response signal output from the plurality of slave units.