1. A sequential X-ray fluorescence spectrometer which irradiates a sample with primary X-rays and measures the intensities of generated secondary X-rays, the spectrometer comprising: a plurality of standard samples to create calibration curves corresponding to analytical samples; and a quantitative analysis condition setting unit configured to preset, as quantitative analysis conditions, with measurement lines for the analytical samples, a half value width of a peak profile of each measurement line, sample constituting elements and their contents of each standard sample, wherein the quantitative analysis condition setting unit performs qualitative analyses of the plurality of standard samples, to obtain, for each of measurement lines identified in each standard sample, a peak angle obtained through peak search, background angles based on the peak angle and the preset half value width of the peak profile, and a net intensity based on a peak intensity at the peak angle and background intensities at the background angles, sets, for each of the measurement lines, the peak angle of the standard sample at which no interfering line is identified within a predetermined angle range centered on the measurement line and the peak intensity is maximum, as a peak measurement angle of the measurement line for the analytical samples in the quantitative analysis conditions, and obtains, for each of the measurement lines, a single virtual profile by synthesizing peak profiles of the plurality of standard samples subjected to the qualitative analyses, and sets the background angle on each of a lower angle side and a higher angle side relative to the measurement line, as a background measurement angle of the measurement line for the analytical samples in the quantitative analysis conditions, the background angles being based on the virtual profile and the preset half value width of the peak profile.