Provided is a dual port Ethernet communication device which allows data received via any one communication port to be bypassed to other communication port. Since the Ethernet communication device performs such bypass operation itself, the communication data can be kept transferred to other terminal unit even though the terminal unit equipped with the Ethernet communication device does not operate normally due to causes such as a break down. In particular, if it is used in each terminal unit forming a closed loop in a closed loop power system, the communication on the closed loop remains such that the adjacent terminal unit can perform backup interrupt function even though a failure happens in a specific terminal unit forming the closed loop.