A compound represented by the following general formula (I) or a salt thereof:
[wherein R1 and R2 represent hydrogen atom, a C1-6 alkyl group, or a group represented by the following formula (A):
[wherein X1 and X2 represent hydrogen atom, or a group represented as —N(R3)(R4) (R3 and R4 represent hydrogen atom, a C1-6 alkyl group, a C1-6 alkylcarbonyl group, or a C1-6 alkyloxycarbonyl group); and n represents an integer of 1 to 6], provided that R1 and R2 do not simultaneously represent hydrogen atom), which is a novel luciferin derivative that serves as a luciferase substrate.