PURPOSE: To provide a data communication system where the data communication is carried out between a terminal equipment and a host device via a 2nd network when a 1st network has a fault and then the 1st network is used again to carry out the preceding data communication after the 1st network is recovered from its fault.

CONSTITUTION: A data communication system includes a network controller 2 which is connected to plural CAT 1-1 to 1-n via the data communication lines, a CAFIS 3 and a card company VAN 4 which are connected to the controller 2, and the host devices 5-1 to 5-m which are connected to the CAFIS 3 and the VAN 4. In a normal state, the VAN 4 is used for communication carried out between the CAT 1-1 to 1-n and the devices 5-1 to 5-m. The controller 2 decides whether the VAN 4 has a fault or not. If the VAN 4 has a fault, the controller 2 switches the network used for data communication to the CAFIS 3. Furthermore the controller 2 detects the recovery of the VAN 4 from its fault. When this recovery is decided, the controller 2 switches the network to the VAN 4 for data communication.