Provided is a titanium-nickel alloy improved in corrosion resistance due to the reduced Ni content in the surface layer. Since a film (modified layer) extremely reduced in Ni concentration compared to the interior can be formed on the surface of a titanium-nickel alloy, by applying electrolysis treatment in a properly controlled electrolytic solution, the corrosion resistance of the alloy is improved. Specific examples of the electrolytic solution may include a mixture of glycerol, lactic acid and water (H.sub.2O).