PURPOSE: To stably control feed water outside a cavitation range, by always supervising a maximum flow rate point which is permitted with regard to the protection of a pump from cavitation.

CONSTITUTION: A feed water flow rate command signal Q2 is applied to a comparator 10 through a low level preference circuit 12. The true rotational frequency of a boiler feed water pump turbine is detected by a detector attached to the turbine and applied as a signal X1 to a function generator 13, A maximum flow rate, which is permitted with regard to the protection of the feed water pump from cavitation while the pump is run, is calculated by the function generator 13 depending on the true rotational frequency signal X1 and applied as an allowable maximum flow rate signal Q2a to one terminal of the low level preference circuit 12. As a result, the output Q2b of the circuit 12 which is entered into the comparator 10, does not exceed the allowable maximum flow rate signal Q2a. The cavitation is thus surely prevented.