A nuclear power plant control system including: a detection unit for detecting a specific event occurring in the nuclear power plant; an on-site equipment handling the event; and a majority decision judging device and an on-site equipment control device which constitute a plurality of control devices respectively operating independently. Each of the control devices includes a plurality of arithmetic units which perform arithmetic processes independently and in parallel based on a detection result of the detection unit, and output a signal for controlling the on-site equipment according to the results of operations of the arithmetic processes. The plurality of arithmetic units perform a matching process for harmonizing process statuses of the arithmetic processes of respective arithmetic units with each other when starting up the control devices, and after completing the matching process, respectively perform the arithmetic processes independently and in parallel.