PURPOSE: To easily detect an abnormality in a gap between the upper mechanism of the core of a reactor and an aggregate of fuel and improve the accuracy of detection by such an arrangement wherein a supersonic generator and receiver is positioned at the same place and caused to generate waves plural times and reflected signals from a supersonic reflector are synchronously added up and then averaged waveforms are to be monitored.

CONSTITUTION: A supersonic generator and receiver 10 is caused to oscillate (n) pieces of supersonic wave. Oscillated supersonic waves are reflected by a supersonic reflector 14 through a gap between a reactor core unit 4 and an upper mechanism 5 of the reactor core unit and they are received by the supersonic generator and receiver 10. The received supersonic waves become such waveforms which are composed of a received signal S of which phase is fixed at all times and random high frequency noises inherent to a nuclear reactor, in other words S1W Sn. For instance, the waveform of a received signal by the first oscillation becomes S+S1 and that of a received signal by the (n)th oscillation becomes S+Sn. By such an arrangement wherein reflected wave signals of n times from the supersonic generator receiver 10 are synchronized in time, the phase of received signal S is adjusted to that of high frequency noise S1WSn and they are added up and averaged, random high frequency noises are reduced although the level of signals from the supersonic reflector is not changed and S/N ratio is improved and a signal Sr can be obtained.