PURPOSE: To obtain the network monitor system which enables the transmission and reception state of messages of packets, converted through a network, to be understood directly and visually.

CONSTITUTION: A remote monitor device is connected to the terminal adapter 5. After terminal adapters 5, and 2 and 3 are connected in remote monitor mode, state information on the packet of a layer 2 or 3 sent to and received from an ISDN network when the terminal adapter 2 or 3 is connected to an opposite-side terminal adapter 3 or 2 is sent to the remote monitor device 6 through the terminal adapter 5 in the form of a B-channel or D-channel packet. The remote monitor device interprets, rearrange, and translates the obtained information according to the transmission and reception time of the packet of the obtained packet state information of the layer 2 or 3, the kind of the packet, and a transmission and reception sequence number, and display the packet state between the terminal adapters 2 and 3 and network at the same time.