A distributed control system includes a plurality of PLCs which are connected together via active and backup transmission lines which are opposite in transmission direction. Each of the PLCs includes a diagnosis command sending program for sending a diagnosis command, via a diagnosis-command transmission line, which is one of the active transmission line and the backup transmission line, to a PLC located downstream on the diagnosis-command transmission line; and an acknowledgment returning program, operable upon receipt of the diagnosis command, for returning an acknowledgement response for the diagnosis command, via an acknowledgment-response transmission line, which is the other of the active transmission line and the backup transmission line, to a PLC located downstream on the acknowledgment-response transmission line.