(1) A novel nucleic acid probe comprising a single-stranded oligonucleotide labeled with a fluorescent dye such that, when the nucleic acid probe is hybridized with a target nucleic acid, the intensity of fluorescence in a hybridization reaction system decreases after a hybridization reaction, and (2) a novel nucleic acid probe comprising a single-stranded oligonucleotide labeled with a fluorescent dye and a quencher substance such that the intensity of fluorescence in a hybridization reaction system increases after a hybridization reaction, the latter oligonucleotide forming no stem-loop structure between bases at positions where the oligonucleotide is labeled with the fluorescent dye and the quencher substance, respectively; various determination kits and DNA chips making use of these probes; method for determining nucleic acids, polymorphous analysis methods and real-time quantitative PCR methods, all of which make use of the determination kits or DNA chips.