Provided is a method for rapidly and easily detecting a mutated nucleic acid, which is contained in a small amount in a nucleic acid sample together with wild-type nucleic acids, with high specificity and high sensitivity. In the method of the present invention, amplification of a detection region comprising a target site by a nucleic acid amplification method is inhibited, by the steps of allowing a nucleic acid having a target site to coexist with a clamp probe comprising a photo-crosslinking nucleic acid and having a sequence complementary to the target site, and photo-crosslinking the nucleic acid having the target site with the clamp probe by photo-irradiation.