Provided is a nucleic acid substrate which has nucleic acid substrate characteristics equivalent to those of dATP, has a low substrate specificity for luciferase, exerts no negative effect on enzymatic reactions such as a complementary-strand synthesis, and therefore is particularly suitable for the pyrosequencing method. As a nucleic acid substrate complementary to nucleotide T, a 7-substituted deoxyribonucleotide triphosphate whose 7-position of a purine group is modified by a substituent is used as a substitute for a nucleotide α-thiotriphosphate analog.