To obtain a detector by which a leaky place in a building is detected nondestructively, quickly and surely by a method wherein a neutron beam source in a trace amount at the legal-regulation-exemption amount or lower of an airtightly sealed radioisotope is used effectively from the outside of the building.

A neutron beam source 1 is arranged in the effective center of a detector. In a detection part, a neutron beam from the neutron beam source 1 is scattered by moisture in a leaky place, and a part of it is detected by a helium-3 detector 2. A signal from the detector is amplified by a preamplifier 3 in a detection part, it is waveform-shaped and amplified by a proportional amplifier 4, and two signals are synthesized by a summing circuit 5 so as to be measured by a pulse-height discriminator 6. A counting operation is performed by using a counting-rate display device 7, a warning generation device 8 and a graphic display device 9 in a display part. In addition, when a moderator 10 is placed at the front of the detector, a leaky place can be detected quickly and surely by the neutron beam source in a trace amount.