PURPOSE: To make a correction for an error due to the differential non-linearity of an A/D converter by making a comparison between a count value derived from the digital signal of the A/D converter and the count value of an ideal pulse height distribution.

CONSTITUTION: An X-ray beam 7 from an X-ray source 1 is incident on a sample 8, and then, a fluorescent X-ray 9 emitted due to the incidence of the beam 7 is incident on the surface 10 of a crystal 5 as an analysis object via a collimator 3. Furthermore. an X-ray beam 11 generated on the surface 10 due to plug reflection arrives at an X-ray detector 6 via another collimator 4. Then, signals detected with the detector 6 are converted to digital signals via an A/D converter 17, and supplied to a memory circuit 19, thereby forming a count value. In addition. the count value of an ideal pulse height distribution and a count value derived from the digital signals are compared with each other, thereby obtaining a correction signal. Then, the count value derived from the digital signals is multiplied by the correction signal. The result of the multiplication is shown on the screen of a monitor 31 as a pulse height distribution, for example, in the form of a histogram.