PURPOSE: To control reactor power with suppressing main parameter variation until moving power station to the own load operation in the case of occurrence of a sudden load reduction event.

CONSTITUTION: A reactor power controlling device gives a load cut generation signal 9 to a control rod drive controller 200, and performs the first power reduction operation in a hydraulic drive mode and the second power reduction operation in an electric force drive mode by giving a power signal from an automatic power controller 100 to a control rod drive controller 200 so that the variation of main parameters is suppressed and the settling power after the load cut is controlled constant. By this, the constant control of the settling power after a sudden load reduction becomes possible and stable power control minimizing the variation of main reactor parameters becomes possible.