It is intended to provide a means and a method whereby a molecule for DNA detection at an elevated accuracy can be provided, different form the existing DNA detection system with the use of a molecular beacon. A molecular beacon wherein fluorescent organic groups capable of forming an excimer are bonded to the 3' and 5' ends of a single-stranded oligonucleotide to be hybridizable with a subject oligonucleotide and the switching of monomer emission/excimer emission is utilized; and a method of detecting SNP by using this molecular beacon.