The remodeling of the neutron irradiation facility of the Musashi Institute of Technology Reactor (TRIGA Mark II, 100 kW) was carried out for the purpose of boron neutron capture therapy. The gamma contamination was reduced by the bismuth scatterer technique, and the thermal-neutron intensity was enlarged by virtue of the cavity effect. A 6LiF sheet was used instead of a 10B sheet for neutron collimation to minimize production of the secondary gamma rays. The characteristics of the optimized field are as follows: ɸth ≈ 1.3 × 1013 m-2· s-1 (1.3 × 109 n/cm2·s), gamma rays ≈ 1.8 × 10-6 O kg-1· s-1 (25 R/h), γ/n ≈ 0.5% in dose equivalent. When a phantom head was placed at the irradiation aperture, the neutron fluence rate (flux) and gamma-ray exposure rate increased to ∼2 × 1013 m-2 · s-1 (2 × 109 n/cm2 · s) and 1.1 × 10-5 C· kg-1 · s-1 (150 R/h), respectively, by the reflection of neutrons and capture gamma rays due to the phantom itself. The facility was licensed by the Japanese government to be used for the medical irradiation purposes on July 20, 1976.