Provided is an apparatus for controlling a gain according to a water level change rate of a steam generator in nuclear power plants. The apparatus includes a water level variance detector detecting a water level variance of the steam generator, a change rate calculator calculating a water level change rate with respect to the detected water level variance, a compensation value calculator calculating a compensation gain value corresponding to the calculated water level change rate, a reactor power sensor sensing whether or not reactor power corresponds to certain power or less, and a gain compensation controller, when the reactor power corresponds to the certain power or less, outputting a control gain value obtained by combining a general gain value provided to control a proportional-integral (PI) controller with the compensation gain value to the PI controller