[en] Materials for a nuclear reactor made of an Ni base precipitation reinforced alloy (inconex X-75) are processed by solid-solubilization. Then, a heat treatment is applied in a reduced pressure atmosphere of 10^-3 to 10² Torr to conduct age-hardening and oxide layer formation. A two-layer structure comprising protection layers of NiFe₂O₄ having good crystallinity and low solubility to high temperature/high pressure water and Cr₂O₃ having an effect of inhibiting diffusion of metal ions are formed on the surface of the base material of Ni base precipitation. Precipitation materials comprising, for example, NiO, Fe₃O₄, γ-Fe₂O₃, TiO and TiO₂ derived from constituent elements of the Ni base precipitation reinforced alloy are contained in the oxide layers. Even if the materials undergo irradiation of neutrons, and ⁵⁸Ni contained in the composition of the materials is converted into ⁵⁸Co, the leaching speed thereof to reactor water is small, thereby enabling to suppress increase of radioactive corrosion products leaching into the reactor water. (I.N.)