PURPOSE: To avoid any scram caused by changing of nuclear reactor pressure by calculating an opening demand signal of a bypass valve based on the actual opening of a regulating valve.

CONSTITUTION: Thermal output of a nuclear reactor is detected by a neutron flux detector 11 and the detected signal 11a is input to a circuit 12 for calculating steam flow rate. A steam flow rate demand signal 12a being output from the calculating circuit 12, together with a frequency controlling signal 14a, are input to a lower value preferential circuit 15 and then an output signal 15a of the lower value preferential circuit 15 is input to a circuit 16 for compensating opening demand. From the compensating circuit 16, opening demand signals 13a and 13a' are output to each regulating valve 3 and 3 to control turbine steam flow rate. Also, a total steam flow rate signal 18a of the turbine is calculated by an adder 19, along with the steam flow rate demand signal 12a and a bypass signal 20, to control a bypass valve 7 by an opening demand signal 13b from the adder 19. With this procedure, scram risk caused by changing of nuclear reactor pressure can be avoided.