PURPOSE: To perform communication between a monitoring device and a host by simple constitution by adding a prescribed header part to the communication data of a LAN monitoring device and providing a function for performing a processing.

CONSTITUTION: In a LAN monitoring system connected to the connection node 3 of an FDDI-LAN by an exclusive interface, the LAN monitoring device 2 is provided with an informing data preparation program 24 and the communication data are edited and generated in a data form requested by the host 1. An LLA protocol processing part 23 transmits a communication frame for which a header for performing the communication by a prescribed protocol between the host 1 is added to the communication data to the connection node. An interface processing part 20 for monitoring is provided with the function for discriminating a data frame for monitoring and a request command frame. Thus, by using the interface for monitoring the LAN by the simple constitution, the communication frame can be transfered between the host 1 without exerting adverse influence on the FDDI-LAN due to the failure of the monitoring device 2.