PURPOSE: To allow the system to recognize the hub connection state without provision of a management function to all hubs by sending/receiving simple management information as polarity information of a transmission packet.

CONSTITUTION: A packet transmission port 1 uses a management information conversion means 13 to convert simple management information of a concerned port into polarity information of each packet based on packet presence information from a carrier detection means 12. A polarity inversion means 14 controls the polarity based on the polarity information. A resultingly generated packet is sent as a form of, e.g. 1001, let a positive packet be information 1 and a negative packet be information 0 as definition. That is, 1-bit data are sent for each packet. A packet reception port 2 uses a polarity information conversion means 24 to detect the management information such as 1001 based on the polarity information detected by its polarity detection means 23. Thus, the connection state is recognized by sending/receiving the simple management information between hubs as a packet polarity change.