PROBLEM TO BE SOLVED: To obtain an EPM determination method capable of accurately determining a plurality of micro-substances containing two or more kinds of same constitutional elements, different in elemental compositional ratio and smaller than a characteristic X-ray generating region at the time of the radiation with electron beam to a practical degree by EPMA. SOLUTION: A micro-substance to be measured is caught on a filter to be irradiated with electron beam and the intensities of characteristic X-rays from two kinds of constitutional elements excited by the irradiation with electron beam are measured and the relative intensities of X-rays of two kinds of these constitutional elements are calculated from the measured intensities of characteristic X-rays and the relative X-ray intensity ratio between two kinds of these constitutional elements is calculated from the obtained relative X-ray intensities. The relative X-ray intensity ratio of two kinds of the constitutional elements in this micro-substance is compared with the threshold value of the relative X-ray intensity ratio of the same constitutional elements for fractional judgment calculated by a Monte Carlo simulation method to fractionally judge the micro-substance and the determination of the micro-substance is performed from the result of this fractional judgment.
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