Abstract: A novel method is provided to assay at least one nucleic acid. According to this method, a nucleic acid polymerization reaction is conducted in a nucleic acid polymerization reaction system, which contains (A) the at least one nucleic acid as a template, (B) at least one nucleotide monomer labeled with at least one label selected from the group consisting of (a) fluorescent dyes, (b) quenchers and (c) immune related substances with a fluorescent dye or quencher contained therein, and (C) at least one nucleic acid-synthesizing enzyme. The template nucleic acid or a nucleic acid, which has been synthesized using the template nucleic acid as a template, is then assayed from a change or an amount of a change in an optical character of the nucleic acid polymerization system.