PURPOSE: To effectively suppress the reactor water level fluctuation when the output is changed by the core flow action.

CONSTITUTION: A reactor water level control device is provided with a feed- forward controller 15a inputted with the output signal URFC13 of a recirculation flow control device 5, estimating the main steam quantity change by the core flow change, and calculating the water feed flow adjustment signal UFF offsetting the main steam quantity change, a feedback controller 16 inputted with the reactor water level signal 7, calculating the drift from the reactor water level preset value, calculating its proportional integral, and outputting the water feed flow adjustment signal UFB, and an adder 18 adding the output signal UFF of the feed-forward controller 15a and the output signal UFB of the feedback controller 16 and outputting the final water feed flow adjustment signal UFWC 17.