The present invention is intended to provide a novel fluorescent labeled single-stranded nucleic acid, by which the background of an exciton oligomer can be further reduced and the novel use thereof. The present invention relates to a labeled single-stranded nucleic acid having at least two fluorescent atomic group pairs that exhibit an exciton effect. The labeled single-stranded nucleic acid is characterized in that the emission peak wavelength of one of the fluorescent atomic group pairs (fluorescent atomic group pair A) is shorter than the excitation peak wavelength of the other fluorescent atomic group pair (fluorescent atomic group pair B), and the fluorescent atomic group pairs A and B have a Förster resonance energy transfer (FRET) effect. This fluorescent labeled single-stranded nucleic acid is usable as a primer for amplifying a target nucleic acid or a probe to be hybridized with a target nucleic acid.