The invention provides a method of selectively amplifying a detection target nucleic acid by inhibiting amplification of a detection non-target nucleic acid (e.g., wild-type gene) in a test sample by using, in a nucleic acid amplification reaction, an oligonucleotide analog containing one or more kinds of one or more unit structures of various nucleoside analogs represented by the following formula (I): wherein the symbols are as defined in the DESCRIPTION, and the like, or a salt thereof, as a clamp nucleic acid, and detecting the amplified nucleic acid.