The present invention provides, for example, a labeling substance that allows the double helix structure of a nucleic acid to be detected effectively. The present invention provides a compound having a structure derived from mononucleoside or mononucleotide, with the structure being represented by the following formula (1), (1b), or (1c), a tautomer or stereoisomer thereof, or a salt thereof. ##STR00001## In the above formulae, B is an atomic group having a nucleobase skeleton, E is an atomic group having a deoxyribose skeleton, a ribose skeleton, or a structure derived from either one of them, or an atomic group having a peptide structure or a peptoid structure, and Z.sup.11 and Z.sup.12 each are a hydrogen atom, a protecting group, or an atomic group that exhibits fluorescence and may be identical to or different from each other.