A multiplex communication system includes a plurality of exchanges and multiplex communication apparatuses connected to at least one exchange through a plurality of lines. Each of the multiplex communication apparatuses comprises first registers for storing first selection number information from the lines and second registers for fetching and storing second selection number information from an associated one of the exchanges, and is arranged such that a CPU of the associated communication apparatus determines that the exchange is to perform its interrupting operation and causes encoders and decoders to be switched to their through mode when the first selection number information stored in the first registers coincides with the second selection number information stored in the second registers.