PURPOSE: To easily manage a concentrated layer removing process in a nondestructive and non-contacting state in a short time by measuring tristimulus values, lightness indexes, or chromaticness indexes and detecting the concentration of a surface concentrating element by comparing the measured index values with calibration curves.

CONSTITUTION: A cold-rolled aluminum alloy plate containing, for example, magnesium is annealed and a magnesium oxide layer formed on the surface of the plate is removed by pickling after cold-rolling. In this process, the tristimulus values X, Y, and X, lightness index L* of color systems L*; a*, and b*, or chromanetic indexes a* and b* are measured by using a lighting optical system following a scattered light vertically receiving system. When the measured values are compared with calibration curves prepared from standard samples having known degrees of surface concentration of magnesium, the concentrating amount of magnesium on the surface of a sample can be detected and the degree of removal of a magnesium concentrating layer can be evaluated.