<p>In a single-stranded nucleic acid having a stem loop structure, an apurinic/apyrimidinic site (AP site) (13) is formed in one nucleic acid strand of a double-stranded stem site (11) which is formed between opposite sequences in the stem loop structure. A ligand (14) capable of binding to a hydrogen and capable emitting fluorescence is added to the single-stranded nucleic acid having the apurinic/apyrimidinic site (13) formed therein, thereby binding the ligand (14) to the apurinic/apyrimidinic site (13).</p>
