PURPOSE: To prevent plural stations whose station number is in duplicate from being subscribed to a link.

CONSTITUTION: Upon the receipt of an existence confirmation frame from a station 0, stations 1, 1' going to subscribe to a link send an existing confirmation reply frame with an identification code specific to its own station added respectively to them (sequence 2). The station 0 adds the identification code in the existing confirmation reply frame received at first to a new station invitation frame and sends the result onto a network. The stations 1, 1' compare respectively the identification code in the received new station invitation with the identification of its own station, a station whose identification code is unmatched (e.g. station 1') is not subscribed to a link and the station number in duplicate is informed to a host (sequence 3). On the other hand, the station whose identification code is matched (e.g. station 1) sends the new station invitation reply frame and subscribes to the link (sequence 4).