To achieve excellent dosimetry of sensitive characteristics to neutrons of a wide range energy.

A thermal neutron detector 20 detects thermal neutrons flying from an external field, and thermal neutrons incident by moderating/reflecting on a human body by middle-high speed neutrons. A high-speed neutron detector 30 detects protons generated by recoil reaction of the high-speed neutrons due to a radiator 32 and simultaneously detects background γ rays. A wave height discriminating circuit 62 discriminates by wave height the detection signal by a lower threshold than completely removing the background γ rays. A detection pulse of the γ rays mixes in output of the wave length discriminating circuit 62, they are removed by discriminating the wave height by a waveform discrimination processing part 70, and only the detection pulse of the high-speed neutrons can be extracted. Since the threshold of wave height discrimination of high-speed neutron detection can be lowered, the neutrons in the neighborhood of 0.01-0.1 MeV in which the conventional sensitivity is worse can be detected to improve sensitivity characteristics.