The present invention provides a firefly luciferase for inexpensive, highly accurate and highly sensitive nucleic acid analysis that uses dATP instead of an expensive reagent having low reactivity to DNA polymerase in the manner of dATPαS, a method of analyzing nucleic acid that uses that luciferase, and a kit for analyzing nucleic acid thereof.The present invention relates to a composition for analyzing nucleic acid that contains luciferase for which reactivity to dATP is equal to or less than 1/400 reactivity to ATP, a method of analyzing nucleic acid that comprises the use of that composition, and a kit for analyzing nucleic acid comprising that composition.