In the case where a template nucleic acid has sequences substantially the same as each other, a primer is designed in a region between these sequences so as to give a product in a ladder structure in which a target region is polymerized in a single sequence via the same sequences in the ICAN reaction. By using a chimeric oligonucleotide primer and a ladder-forming oligonucleotide primer containing a specific base sequence, an amplification product having a ladder structure can be positively formed and thus the sensitivity, amplification efficiency and reaction speed in the ICAN reaction can be elevated.