PURPOSE: To reduce the number of reference materials and to apply the title method also to test materials of types different from a reference material by a method wherein a diffracted X-ray intensity measured value corresponding to the interval between the same crystal faces of the test material and the reference material and the theoretical intensity formula of diffracted X-rays are used.

CONSTITUTION: X-rays are radiated, so as to be at a prescribed incidence angle α, to an alloyed molten zinc-plated steel plate 58 by a Cr target X-ray tube 57. Counting tubes 52, 53, 54 which are installed at angles corresponding to individual alloy phases (δ1, ξ, Γ) constituting a plated layer, counting tubes 51, 55 for background X-rays on the low-angle side and the high-angle side and a counting tube 56 for diffracted X-rays on an α-Fe face are installed with reference to X-rays which are diffracted by the steel plate 58. Then, an X-ray generation apparatus and a cooling-water supply apparatus which cools the X-ray tube 57 are connected to a measuring head 50, an operation and processing apparatus is connected additionally, a background is deduted automatically on the basis of values counted by the individual counting tubes 51 to 56, a relative intensity R with reference to a reference material is computed, and the thickness of every alloy phase is computed.