A method is described for detecting a target nucleic acid, in which the target nucleic acid is amplified in the presence of a proofreading DNA polymerase and an allele-specific reactive primer (ASRP) including (i) a nucleotide sequence complementary to the target nucleic acid and (ii) modified nucleotide(s) located in a region from a nucleotide located right before a non-complementary nucleotide on the 5′-side of the non-complementary nucleotide, so that an amplification product of the target nucleic acid is produced when the ASRP is complementary to the target nucleic acid, and so that when the ASRP is not complementary to the target nucleic acid, an amplification product of the target nucleic acid will not be produced. The detection method has a very high specificity of amplification, and is useful for applications such as detecting mutations, detecting methylation of CpG after bisulfite treatment, and detecting target DNA from a DNA library.