A novel nucleotide derivative, in case of existing as a member of a single-stranded sequence, undergoing a change in the fluorescent signal intendity depending on the corresponding base type in the partner strand with which the single-stranded sequence is hybridized, and which is a thymin/uracil derivative (1) emitting light most intensely when a confronting base in the partner strand with which the single-stranded nucleotide sequence is hybridized is adenine; a cytosine derivative (2) emitting light most intensely when the confronting base is guanine; an adenine derivative (3) emitting light most intensely when the confronting base is cytosine; a guanine derivative (4) emitting light most intensely when the confronting base is cytosine or thymine/uracil; and an adrnine derivative (5) emitting light most intensely when the confronting base is thymine/uracil/.