[en] Purpose: To detect the counterflow state of a recirculation pump as a differential pressure, and after confirming the detection result, and operation circuit is switched, whereby the fluid flow in the core can be accurately detected even at a time when one recirculation pump is stopped. Method: A method of measuring the fluid flow in the core of a bwr type reactor by seeking the total flow of fluid flows of jet pumps belonging to A and B loops so as to obtain the fluid flow in the core of the reactor, which method is characterized by subtracting the flow of the stopped loop jet pump from that of the sound loop jet pump in accordance with the compatible establishment of a pump stop condition and such a condition that the differential pressure of the flow generator of the jet pump of the stopped loop is less than the predetermined value at a time when one loop is tripped. (Nakamura, S.)