PURPOSE: To serve a purpose with only one serial communication interface in an intermediate communication equipment by providing a transmission route connector at the side of the transmission output of the intermediate communication equipment.

CONSTITUTION: Communication data to the second communication equipment 12, for example, which is transmitted from the first communication equipment 11 is received in the communication equipment 12. Its microcomputer 30 confirms that received communication data is transmitted to the communication equipment 12 where the microcomputer itself belongs to. The transmission route connector 27 connected with a transmission output terminal 20 is executed connection. Thus, duplex communication between the communication equipments 11 and 12 is made to be possible. The microcomputer of the third communication equipment 14 confirms that received communication data is not transmitted to the communication equipment 14 where the microcomputer itself belongs to and keeps the transmission route connector 28 apart.