The present invention provides an in situ monitoring of the decomposing activity against oligonucleotides target in a biological tissue. A single-chain oligonucleotide target comprises an appropriate number of nucleic acid bases to be examined with an energy donor and an energy acceptor respectively at its 5'- and 3'-terminals. Monitoring of the fluorescence changes of the target after injection into a biological tissue, particularly the fluorescent resonance energy transfer(FRET) phenomena between the energy donor and acceptor, indicates whether the oligonucleotide is not decomposed yet.