PURPOSE: To perform highly accurate determination by an X-ray diffraction method, by measuring a ratio of two diffraction line intensities, determining plating composition, computing a correcting attenuation ratio (x) from a plurality of diffraction line intensities having different lattice spacing with respect to α-Fe that is a steel plate basis under a plated layer, and finding the amount of the attached plated material.

CONSTITUTION: Good correlation exists between a diffraction line intensity ratio Iδ1/Iγ and the amount of Fe. The ratio of two diffraction line intensities in δ1, γ, or ξ phase is measured. The amount of Fe or the amount of Zn in a plating material is obtained from the value or the ratio. Ratios I/I° are obtained for a plurality of diffraction plane indexes (hkl)i, where I is the diffraction line intensity with respect to α-Fe of the steel plate basis of the plated steel plate, and I° is the diffraction line intensity with respect to a bare uniform steel plate, whose diffraction plane index corresponding to I is the same. Stable correlation is shown between the added and averaged value of the ratios, i.e., a correcting attenuation ratio (x), and the amount of the attached plating material. When the value of (x) and the determined value of the plating composition are used, the amount of the attached plating material can be determined accurately.