An object of the present invention to provide a firefly luciferin and firefly luciferin analog that are modified to maintain luminescent activity by luciferase in a firefly bioluminescent system. In particular, it is an object of the present invention to provide a new luminescent substrate for which the emission wavelength in a firefly bioluminescent system is shifted to a longer wavelength than that of a conventional luminescent substrate. The present invention provides a luciferin in which the benzothiazole ring moiety has been modified at the 7-position, a luciferin analog in which the benzene ring moiety has been modified at the 6-position, and a luciferin analog in which the 6-(dialkylamino)-2-naphthalenyl moiety has been modified at the 5-position.