PURPOSE: To minimize a water level change of a nuclear reactor and at the same time, to enable quick switching of feed water pumps, by connecting both feed pumps to a main feed water controller and by imparting inverse bias values to the both pumps simultaneously.

CONSTITUTION: Signals from a water level detector 100 and a water level setter 120 of a nuclear reactor are fed to an accumulator 110 and then their deviation signal is fed to a main feed water controller 130 to output a command signal to a feed water pump. After that, a signal of a bias value setter 410 of the first feed pump is fed to an accumulator 400 to control a feed water flow rate of the first feed water pump 300 via a controller 320 of the first feed water pump. In the same manner, a feed water flow rate of the second feed water pump is controlled via a controller 321 of the second feed water pump. In this occasion, when two inverse commands for the same flow rate are fed to each of the both feed pumps, respectively, the feed pumps can be switched without generating any water level changes being accompanied.