A water level control system for enhancing the controllability of the level of water retained in a steam drum. A water level control section of the system determines the volume of retained water in a steam-water separator from a detected steam flow rate signal by using a function operator and a first order lag circuit, determines a signal representing the mass flow rate balance of water in the steam-water separator by differentiating the volume of retained water with respect to time with a differentiator, generates a compensated signal by adding the signal to the difference between a detected feed water flow rate signal and the detected steam flow rate signal, and properly compensates a valve area demand for a feed water flow control valve.