In the present invention, a reactor power control apparatus of a natural circulation reactor comprises a power control apparatus for controlling reactor power and a pressure control apparatus for controlling reactor pressure. A power adjustment demand signal S4 is input from the power control apparatus into the pressure control apparatus. The pressure control apparatus controls degree of opening of a inlet port steam control valve provided in a moisture separation heater that is connected to a low-pressure turbine, based on the power adjustment demand signal S4.A reactor power control apparatus of the natural circulation reactor can suppress reactor power variation and obtain stable power.