[en] The thermal neutron column of the JRR-2 reactor was remodelled in order to install a medical irradiation facility. The design experience and the cource of this remodelling are presented. At first, the thermal neutron flux was lower than expected, and the expected radiation condition was achieved by the following improvements: 1) removal of graphite from the thermal neutron column to increase the neutron flux, 2) twice increase of bismuth shielding and the shift of its position to improve the shielding effect against gamma ray, 3) application of B₄C rubber on the both surfaces of the shielding door to suppress the secondary gamma ray generation from the structural materials, 4) LiF tiles are applied on the inner face of the neutron beam exit hole to suppress the secondary gamma ray from the beam exit, and 5) installment a cone-shape polyethylene collimator to relieve the decay of thermal neutron. After the improvements, medical irradiation treatments have been performed successfully since August 10, 1990. (T.H.)