An object of the present invention is to shorten the time required by processing, and to simplify the work and increase the number of samples processed, in the X-ray analysis of a specific substance contained in a sample. To achieve the object, a method for assaying the content of a specific substance in a sample 106, comprises a measurement step (S3) of irradiating the sample 106 with X-rays, and measuring the spectrum of the sample 106, and an assay step (S4) of comparing the components of the specific substance in the spectrum of the sample 106 and in a reference spectrum that has been pre-stored for the sample 106, and determining whether the specific substance is contained in the sample 106 in an amount equal to or greater than a specific value.