PURPOSE: To prevent cavitation by comparing a jet pump and a recirculating pump with the pressure and flow characteristic curves of both pumps and running back recirculating pump when the measured values are lower than the curves.

CONSTITUTION: Individual pump flows from a jet pump 2 enclosed in an atomic reactor pressure container 1 and a recirculating pump 7 arranged in a recirculating pipe 6 connected to this container 1 are measured by flow measuring devices 20, 21 to generate flow signals 20a, 21a. The flow signal 20a detected by the jet pump 3 and the pressure signal 22a detected by the recirculating pipe 6 are fed to a comparator 24 and are judged whether to meet the jet pump characteristic A. When the pump characteristic is deteriorated from a dot (.) indicating a normal detection value to a circle () indicating a cavitation occurrence, a recirculating pump runback signal 25 is generated by the comparator 24. Accordingly, the recirculating pump 7 quickly runs back to prevent a cavitation occurrence.