An X-ray fluorescence spectrometer which includes a calculating device (10) operable to calculate the theoretical intensity of secondary X-rays (6), emanated from each of elements contained in a sample (13), based on the assumed composition and then to successively approximately modify and calculate the assumed composition so that the theoretical intensity and the converted and measured intensity, which have been detected by a detecting device (9) and then converted in a theoretical intensity scale, can match with each other, to thereby calculate the composition of the sample (13). The calculating device (10), when calculating the theoretical intensity, performs a simulation to determine the theoretical intensity of the secondary X-rays (6) for each of optical paths, using the size of the sample (13), and the intensity and the incident angle (φ) of primary X-rays (2) impinged upon various areas of the sample surface (13a) as parameters.