The present invention provides a fluorescent substance which is represented by a formula: A-B-C wherein A is a residue of natural or synthetic nucleotide, oligonucleotide, polynucleotide, or derivative thereof, and binds to B at a base moiety in said residue, or A is a residue of avidin or streptavidin; B is a divalent linking group or a single bond; and C is a monovalent group derived from a general formula (I) and binds to B at a reactive group present in R1 or R2: wherein R1 and R2 each independently represent an alkyl group that may be substituted with a reactive group capable of covalently bonding to A-B-; R3, R4, R5, and R6 each independently represent an alkyl group, and R3 and R4, and/or R5 and R6 may bind to each other to form a saturated carbon-ring together with a carbon atom(s) to which they bind; V1, V2, V3, V4, V5, V6, V7, V8, V9 and V10 each independently represent a hydrogen atom or a monovalent substituent, and two adjacent groups thereof may bind to form a ring; L1, L2, and L3 represent a substituted or unsubstituted methine group; each of m, n, s, and t represents 0 or 1, provided that m+n=1 and s+t=1; p represents 1, 2, or 3; M represents a counter ion; and q represents a number required to neutralize the charge of a molecule. The fluorescent substance of the present invention is useful as a labeling substance for nucleic acids, or as a reagent for analyzing biological components such as nucleic acids, proteins or sugars.