PURPOSE: To realize a mis-connection detector able to detect misconnection between a transmission line and a reception line in the line concentrator for a LAN using a twist pair wire.

CONSTITUTION: The detector consists of a differential signal reception means RX detecting a signal from an output line of a transmission section of each port, a relay flag signal output means FG outputting a relay flag signal representing it that the line concentrator is relaying data, and an AND means AN outputting an AND output between an output of the differential signal reception means RX and the relay flag signal as a mis-connection detection signal. Thus, the sure detection of mis-connection for the line concentrator is attained through the use of the relay flag signal by having only to add the simple means such as the differential signal reception means RX and the AND means AN.