The present invention provides a primer that effectively can detect, for example, the double helix structure of a nucleic acid. The primer is a labeled nucleic acid containing at least one structure represented by the following formula (16), where 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 are each an atomic group that exhibits fluorescence and are identical to or different from each other.