1. A concentration measuring method for measuring a concentration of an object component in a sample from a fluorescent X-ray intensity by means of a fluorescent X-ray spectrometer, comprising steps of: acquiring spectrums of secondary X-rays generated by irradiating a plurality of standard samples, concentrations of the object component of which are zero and compositions of which are different from each other, with primary X-rays; setting a subtraction formula by calculating constants α and β according to a plurality of formulas obtained by setting I=0 and assigning S and B, which are obtained from the spectrums acquired for a plurality of standard samples, to a subtraction formula I=S−αΔB−β (wherein I: a fluorescent X-ray intensity obtained by subtracting the background, S: a fluorescent X-ray intensity of the object component obtained from a spectrum, α and β: constants, and B: a scattered X-ray intensity obtained from a spectrum) for subtracting the background from the fluorescent X-ray intensity of the object component, which is obtained from a spectrum; irradiating the sample with primary X-rays; acquiring a spectrum of secondary X-rays generated from the sample; obtaining a fluorescent X-ray intensity of the object component included in the sample from the acquired spectrum; subtracting the background from the fluorescent X-ray intensity of the object component by assigning S and B, which are obtained from an acquired spectrum, to a subtraction formula in which calculated values are set as α and β; performing a correction corresponding to a composition of the sample for the fluorescent X-ray intensity obtained by subtracting the background; and calculating a concentration of the object component in the sample on the basis of a calibration curve representing a relation between a concentration of the object component and a value obtained after performing the correction for the fluorescent X-ray intensity obtained by subtracting the background.