Abstract: A method in which a surface layer material whose amount of reduction is to be measured is activated to find a distribution ratio of two or more kinds of radioactive nuclides that are produced in the space in the surface layer of the material, and the distribution ratio is used as an index for the amount of reduction of surface layer material. The amount of reduction in the surface layer material is measured in situ, nondestructively, easily and irrespectively of obstacles and the distance that exist between the detector and the material that is to be measured.