To facilitate a maintenance operation by allowing a master station to perform danger-signal detection processing which notifies a danger signal of a remote station and a transmission cable, based on an occurrence state of life detection time-out of each remote station counter.

Each remote station of plural remote stations 1, 2,... is separately provided with a counter which counts a number of time-out occurrences to polling, is cleared in a prescribed cycle and also decides life detection time- out when it reaches a prescribed count value. A master station decides waste and degradation of singular or plural remote stations and transmission cables, based on an occurrence state of life detection time-out for each remote station counter and performs danger signal detection processing which notifies a danger signal, in response to this decided content. When the danger signal is received, the remote station or transmission cable which is designated by the danger signal is inspected and replaced by using a man-machine device, etc., which is connected to a CPU of a sequencer.