The neutron irradiation apparatus 100 includes an introduction tube 1 for introducing a proton beam, a target structure 2 provided in a lower end of the introduction tube 1, an aluminum fluoride layer 3 disposed below the target structure 2 in an irradiation path of neutrons generated in the target structure 2, and a heavy water layer 4 placed under the aluminum fluoride layer 3 in layers. The aluminum fluoride layer 3 is set at a thickness that increases epithermal neutrons. Since use of only the aluminum fluoride layer 3 increases its thickness too much, heavy water is placed. Heavy water moderates neutrons quickly, and allows increasing epithermal neutrons without increasing the thickness. The combination of the aluminum fluoride layer 3 and the heavy water layer 4 allows increasing epithermal neutrons by attenuating only fast neutrons without increasing thermal neutrons. Accordingly, neutron flux with many epithermal neutrons is obtained.
