a pair of controllers each collecting input data from a controlled subject, and outputting control data based on the input data;

a plurality of remote input/output units each connected to both of the pair of network loops in the single loop network for collecting the input data from the controlled subject, transmitting the input data to both of the controllers, and outputting, to the controlled subject, the control data transmitted from an active one of the pair of controllers currently executing control;

communication cables each connecting the pair of controllers and the plurality of remote input/output units to make the pair of network loops in parallel; and

a pair of redundant control interface units for mutually monitoring abnormality statuses of the pair of controllers, and setting a control right allowing any one of the pair of controllers to execute control in an active state, and the other controller to be in a standby state, and

the redundant supervisory control system being configured to control the controlled subject by performing communications in two directions of clockwise and counterclockwise in each of the pair of network loops in the single loop network composed of the pair of controllers, the plurality of remote input/output units and the communication cables, in accordance with a predetermined communication protocol,

wherein each of the pair of controllers includes:

a pair of first loop interface units each communicate the input data and the control data in the two directions with the counterpart controller and the remote input/output units, and judge whether the input data is normally received;

a first switching controller which selects, among the pair of first loop interface units, one of the first loop interface units which is capable of performing normal reception, based on results of the judgments of the respective first loop interface units;

a first data distributor-selector which receives the input data outputted from one of the first loop interface units selected by the first switching controller, and distributes the control data among the pair of first loop interface units; and

a control computation unit which receives the input data outputted from the first data distributor-selector, performs arithmetic processing on the input data, and outputs, as the control data, the result of the arithmetic processing to the first data distributor-selector, and

wherein each of the plurality of remote input/output units includes:

a pair of second loop interface units each communicate the input data and the control data in the two directions through each one of the pair of network loops with the pair of the controllers, and judge whether the control data is normally received;

a second switching controller which selects, among the pair of second loop interface units, one of the second loop interface units which is capable of performing normal reception based on results of the judgments of the second loop interface units;

a second data distributor-selector which outputs the control data outputted from one of the second loop interface units selected by the second switching controller, and distributes the input data collected from the controlled subject among the pair of second loop interface units; and

an input/output unit which transmits the control data outputted from the second data distributor-selector to the controlled subject, and transmits the input data from the controlled subject to the second data distributor-selector, whereby

when a reception failure is detected in at least one portion on one of the pair of network loops including one or both of the pair of first loop interface units and one or both of the pair of second loop interface units in one or plural remote I/O units, one or plural first loop I/F units capable of normally communicating and one of the pair of second loop I/F units in one or plural remote I/O unit, capable of normally communicating are automatically selected, so that transmissions are performed in one or both transmittable directions of each of the pair of network loops; and

in a case where plural failure portions of the respective network loop do not overlap with each other, each of the portions starting from each of the pair of controllers, and ending at a location of a failure, the control is able to be continued without switching the control rights.