A novel nucleotide derivative, in case of existing as a member of a single-stranded sequence, undergoing a change in the fluorescent signal intensity depending on the corresponding base type in the partner strand with which the single-stranded sequence is hybridized, and which is (1) a thymine/uracil derivative emitting light most intensely when a confronting base in the partner strand with which the single-stranded nucleotide sequence is hybridized is adenine; (2) a cytosine derivative emitting light most intensely when the confronting base is guanine; (3) an adenine derivative emitting light most intensely when the confronting base is cytosine; and, (4) a guanine derivative emitting light most intensely when the confronting base is cytosine or thymine/uracil.