In this paper, we introduced a fault tolerant control system with the aim of achieving higher degree of reliability for a PLC control system in the field network. The system reliability was evaluated by MTBF(Mean Time Between Failure). The design of the fault tolerant system through CC-Link of Mitsubisi's MELSEC network was presented. In addition, the PLC data is transmitted from the field network's PC to the host PC by TCP/IP Window socket.