PURPOSE: To provide a network system which can keep a comparatively high working rate even when a fault or the like is generated at a connected line.

CONSTITUTION: The network system is provided with a network 6, host systems 1 and 2 connected to the network 6 so as to transmit data each other while establishing a communication path, plural terminal systems 3 and 4, and managing device 5 connected to the network 6. The managing device 5 checks data to be transmitted on the network 6 and when the communication path is cut off without transmitting data from the host systems 1 and 2 to the terminal systems 3 and 4 although the data are transmitted from the terminal systems 3 and 4 to the host systems 1 and 2 after the communication path is established between the terminal systems 3 and 4 and the host systems 1 and 2, abnormality is judged.