PURPOSE: To make quickly analysis online and to utilize the result thereof for controlling the thickness and composition of plural layered plating film of a pure metal or alloy by subjecting the coating amt. and compsn. of the film to fluorescent X-ray analysis after the formation of the respective films and performing the measurement for the 2nd layer in the position corresponding to the measuring position of the 1st layer.

CONSTITUTION: The plating of a pure metal such as Zn or a metal or an alloy having different compsn. such as Fe-Zn on, for example, with a substrate steel plate 1 is accomplished by plating first a layer 11 then making an exciting X-ray incident to the plating film at the low incident angle ΨL1 at which the fluorescent X-ray from the metal 1 is not detected, detecting the intensity of the fluorescent X-ray at a taking out angle ΨL1 and analyzing the compsn. The intensity of the fluorescent X-ray at ΨH1 is then detected by exciting X-ray at the incident angle ΨH1 higher than ΨL1 and the coating amt. (film thickness d1) thereof is measured. The 2nd layer 12 is then plated on the first layer and the compsn. is analyzed by the low incident angle ΨL2 at which the fluorescent X-ray of the layer 12 is not measured and the taking out angle ΨL2. The coating amt. d2 is then measured by high angles ΨH2, ΨH2. The quick analysis is thus made possible online with good accuracy.