A duplexed control system comprises: two control devices, whereof one is set to operate as a reserve system and the other is set to operate as a regular system; a tracking bus that connects the two control devices to enable these to exchange tracking signals; and a control transmission channel that transmits computational information of these two control devices; wherein: the control devices respectively have: an own-system state evaluation section that transmits the result of state evaluation of its own system as a tracking signal through the tracking bus to the other system; an other-system signal confirmation section that receives the tracking signal through the tracking bus from the control device of the other system and thus performs state confirmation; and a transmission channel changeover section that, if the tracking signal cannot be received through the tracking bus, effects transmission of the tracking signal to the other-system signal confirmation section of the other system, through the control transmission channel.