PURPOSE: To allow an X-ray of a single wavelength to enter a sample by disposing a monochromometer between an X-ray tube and sample.

CONSTITUTION: The monochromometer 11 consisting of, for example, a solar slit 9 and spectral crystal 10 is provided between the X-ray tube 1 and the sample 5. Only the X-ray of the direction determined by the slit 9 is thereby selected from the X-rays 2 generated from the tube 1 and such X-ray enters the crystal 10. The X-ray is diffracted by the crystal 10 and only the X-ray 2 of a desired wavelength enters the sample 5. The incident angle of the X-ray 2 can be changed and the wavelength thereof can be changed by rotating the crystal 10 in an arrow (b) direction. The fluorescent X-ray 6 generated from the sample 5 is screened by the solar slit 3 and is diffracted by the spectral crystal 4. The intensity of only the required X-ray 6 is measured by a detector 5. The incidence of the X-ray having the single wavelength is executed according to the above-mentioned device and therefore, the absorption coefft. of the element to be measured unequivocally determined with good accuracy.