Provided is a DNA-level grain variety discrimination method of detecting the presence or absence of any other varieties of grains in object grains of a certain variety through multiplex PCR that uses the DNAs extracted from the grains or from their processed products as templates. The method is characterized in that the multiplex PCR uses pair primer groups that are for discriminative detection of negative bands not appearing in the band pattern of the object variety but selectively appearing only in the band patterns of the other mixed varieties. The method has made it possible to rapidly and simply detect the presence or absence of mixed varieties in high-quality grains such as “Koshihikari”, and to identify the mixed varieties.