In a CATV-LAN in which a plurality of modems transmit messages asynchronously from respective terminal apparatuses to an upstream transmission path and receives messages from a downstream transmission path, when a modem is transmitting a message, the resultant data which are received by that modem are compared with data of that message which have already been transmitted, and transmission is interrupted if the received data do not correctly match the transmitted data, due to conflict with data transmitted by some other modem. Each message contains information specifying its length, and when a non-transmitting modem receives a set of data whose actual length does not match a length value which is specified within the data, the data are discarded. It thereby becomes unnecessary to provide beat detection circuits in the modems, for detecting data conflict, so that system cost can be reduced.