To lighten the load of abnormality monitoring and a switching process on a controller and to shorten the switching time by allowing a stand-by system remote input/output device to autonomously monitor abnormality of an in-use system remote input/output device and switch input/output processes.

The stand-by system remote input/output device 13b monitors report information on abnormality occurrence that the in-use system remote input/output device 13a sends out to a serial transmission line 5 through a communication process part 7b. Then an in-use system abnormality monitor part 14 detects the abnormality of the in-use system remote input/output device 13a from its contents. In response to this detection, the in-use system abnormality monitor part 14 turns on an input/output process switching part 15b to enable the communication process part 7b and stand-by system input/output process part 9b to exchange data with each other, and starts communicating with a controller 11 to perform an input/output process with process equipment 10 instead of the in-use system remote input/output device 13a.