Disclosed are: a firefly luciferase for use in the inexpensive, highly accurate and highly sensitive nucleic acid analysis using dATP but any expensive reagent having poor reactivity to DNA polymerase such as dATPαS; and a nucleic acid analysis method using the firefly luciferase; and a nucleic acid analysis kit. Specifically disclosed are: a composition for use in the analysis of a nucleic acid, which comprises a luciferase whose reactivity to dATP is 1/400 or less relative to its reactivity to ATP; a method for the analysis of a nucleic acid, which is characterized by using the composition; and a kit for the analysis of a nucleic acid, which comprises the composition.