To make it possible to communicate one PC when the other PC becomes abnormal in the case that the doubled PCs are connected to an Ethernet transmission line.

The doubling I/F part of a PC1 consists of an AND circuit 11 and a 1st OR circuit 12 and the AND circuit is supplied with UDP information from a monitor device DCS as its 1st input terminal and with a signal from a PC2 at its 2nd input terminal. The normal signal of the PC2 is supplied to the PC2 through the 1st OR circuit 12. The 1st OR circuit 12 is further supplied with a signal showing the PC1 is abnormal. The doubling I/F part of the PC2 comprises 2nd and 3rd OR circuits 13 and 14, and UDP information from DCS, UDP information from the PC1, and a PC2 abnormality signal are inputted to the 2nd OR circuit 13. The 2nd OR circuit 13 supplies one of those signals to the 2nd input terminal of the AND circuit 11. The 3rd OR circuit 14 is supplied from the output signal from the 1st OR circuit 12 and a PC1 abnormality signal and when those signals are present, the transmission right is switched from the PC1 to the PC2.