PURPOSE: To eliminate time delay and to find accurate core performance by providing neutron flux measuring instruments which move axially in a nuclear reactor and a neutron flux monitor for monitoring neutron flux in the nuclear reactor.

CONSTITUTION: The moving neutron flux measuring instruments 3-1-3-n move up and down in TIP waveguides 2-1 - 2-n to detect neutron flux and outputs neutron flux detection signals. Those signals are sent to a neutron flux probe monitor 5, which receives the respective signals to monitor the neutron flux in the nuclear reactor 1 and converts the respective current signals into current signals, which are sent to respective neutron flux driving controllers 10-1 10-2. Position detectors 4-1 4-n, on the other hand, move up and down in the waveguides 2-1 - 2-n integrally with the measuring instruments 3-1 - 3-n which move respectively and output their position detection signals. Position detection signals of 1-3 channels are sent to a synchronous converter 7-1 and position detection signals of 4 and 5 channels are sent to a synchronous converter 7-2 and converted into digital signals, which are sent to the controllers 10-1 and 10-2, so that data packages are sent out to a process computer 11.