PURPOSE: To complete the meaningless re-transmission of multi-address data and to prevent communication efficiency on a local area network (LAN) from being lowered by issuing a false response to the multiple address data re-transmitted frequently.

CONSTITUTION: A LAN monitoring means 3 monitors the multiple address data received from the LAN and response data for that via a LAN communication controller 2, and stores the transmission origin node information of each mltiple address data, the number of times of re-transmission, and response request destination node information on the LAN. Furthermore, the means 3 monitors the communication efficiency of all the data on the LAN, and instructs the transmission of false response data to a false response means 4 by detecting the multiple address data for a response request node in which the number of times of re-transmission exceeds a prescribed value when the communication efficiency goes less than a regulated value. The means 4 generates the false response data including the medium access control address of a LAN fault intervening device 1 itself, and transmits it to a transmission origin node. Thereby, the re-transmission of the multiple address data can be completed, which prevents the communication efficiency on the LAN from being lowered.