Provided are the following: a method, for improving reactivity of a nucleic acid synthesis reaction, comprising a step for adding an ω-amino acid to a reaction solution; a composition, for a nucleic acid synthesis reaction, comprising DNA polymerase, reaction buffer, at least one primer, at least one deoxyribonucleotide triphosphate, and an ω-amino acid; and a reaction buffer, for a nucleic acid synthesis reaction, comprising an ω-amino acid.