A method for detecting species in a target plant genus comprises the steps of conducting PCR using at least one member selected from the group consisting of primers (A) and (B), which can hybridize under stringent conditions to a nucleic acid molecule having a common nucleotide sequence for all species in the target plant genus in 45S rRNA precursor gene sequence thereof, wherein 3' end of primer (A) can complementarily bind to a base in ITS-1 sequence of the target plant genus when the primer hybridizes to the nucleic acid molecule while 3' end of primer (B) can complementarily bind to a base in ITS-2 sequence of the target plant genus when the primer hybridizes to the nucleic acid molecule, and identifying the presence of the resulting amplification product from PCR containing at least a part of ITS-1 or ITS-2 sequence of the target plant genus. The method for detecting species in a target plant genus, particularly an allergenic plant genus such as the genus Fagopyrum, can make it possible to detect with high sensitivity, for example, about 1 ppm of the plant(s) in cases where the plant(s) is contained in a food ingredient or food product.