[en] Neutron capture therapy was restarted at the heavy water thermal neutron irradiation facility on February 1990. Control of the energy spectrum of incident neutrons is examined as a method to improve thermal neutron flux distribution in the depth of a body. Hyper-thermal neutron is defined as neutron which can be approximated with Maxwell distribution of higher temperature than 300K. A region of 0.1 to 3 eV is regarded to be the hyper-thermal region. Thermal neutron flux distribution in vivo with the hyper-thermal neutron irradiation is calculated by simulation with MCNP-V3 code. Scattering material to obtain hyper-thermal neutron is examined. The results of the study show that the reaction rate is improved by the use of hyper-thermal neutron to enable treatment of the tumor 2 cm deeper than the previous thermal neutron field, ZrH_1.7 at 1200 K is appropriate as a scattering material and the use of Bi layer can eliminate mixing gamma ray without deterioration of the irradiation field. (T.H.)