PURPOSE: To hold the secrecy of a data packet by permitting a special packet to correspond to every line concentrator and a port, transmitting the special packet from terminals on a network and designating and controlling the line concentrators and the ports.

CONSTITUTION: Preamble length detection circuits for detecting the special packet composed of a preamble and the like are provided for the line concentrators 21-23, for example. Then, the special packet is made to correspond to respective line concentrators 21-23 and the ports, and the special packet is transmitted from the terminals 24-30 on the network. Thus, the line concentrators 21-23 are designated and controlled. Thus, the line concentrators 21-23 and the ports are designated and connection/detachment with the network can selectively be executed. Then, the packet is not transmitted to the specified terminal in the terminals 24-30 on the network. Consequently, the secrecy of the data packet can be maintained.