Provided is a method of quantifying a specific plant genus in a food or a food ingredient by a PCR method, comprising: (i) preparing a sample for correction where a sample derived from the specific plant genus to be detected and a standard plant sample are mixed in a predetermined ratio, and extracting genomic DNA from the sample for correction; (ii) preparing a test sample where a known amount of the standard plant sample is added to the food or the food ingredient to be examined, and extracting genomic DNA from the test sample; (iii) practicing a quantitative PCR method using the genomic DNAs and primers; and (iv) conducting correction with a standard value for correction determined for the sample for correction to calculate the amount of the specific plant ingredient contained in the test sample.