To solve the problem that a proper neutron spectrum cannot be found because the measuring condition of each neutron detector is not identical when the size of the neutron detector having the different thickness of a moderator used to measure different neutron energies is different in such a way that the distribution of a neutron flux is not uniform.

The neutron detector 1b is constituted in such a way that a support member 5b is installed at the outside of a thermal neutron detector 2 and that a thermal neutron absorber 4 is installed additionally at the outside of the moderator 3b. The thickness of the support member 5b is adjusted so as to match the thickness of the moderator 3b, and the size of the neutron detector 1b is made identical to the size of other neutron detectors in the neutron spectrum measuring device. Thereby, irrespective of the thickness off the moderator, the size of the neutron detectors becomes identical, and the proper neutron spectrum can be found and measured.