To provide a method for identifying a nucleic acid capable of being concerned with a triple helix formation by using at least two adjacent annealed probes by combining excellent discriminability of mismatch, which is characteristic of a small sequence, with statistical specificity of a longer nucleo-base sequence, and further to obtain a composition containing the nucleic acid and the probe, and a composition containing the nucleic acid, the probe and other third probes.

This method for identifying nucleic acid A having adjacently bonded sequences A1 and A2 comprises (a) a step for bringing the nucleic acid A into contact with the first nucleic acid analogue B having the sequence B1 complementary to the sequence A1, and the sequence B2 capable of being concerned with a triple helix structure, and the second nucleic acid analogue having the sequence C1 capable of being concerned with the triple helix structure and the sequence C2 complementary to the sequence A2, and (b) a step for measuring formation of the complex containing the nucleic acid A, the nucleic acid analogue B and the nucleic acid analogue C.