It is intended to provide a novel monomer unit by which Z type DNA can be more effectively stabilized, a reagent for integrating this monomer unit into an oligonucleotide, and a method of stabilizing Z type DNA by using the reagent. Namely, a guanosine derivative represented by the following general formula [1]: ##STR00001## wherein R.sup.1 represents acyl; R.sup.2 represents lower alkyl; R.sup.3 represents tri-substituted silyloxy or tetrahedropyranyloxy; and R.sup.4 represents cyanoethyl or allyl; a reagent for stabilizing Z type DNA which contains the guanosine derivative; and a method of stabilizing Z type DNA by using the guanosine derivative. It is also intended to provide a method of transferring guanosine having lower alkyl at the 8-position into an oligonucleotide by using the guanosine derivative; and an oligonucleotide carrying guanosine having lower alkyl at the 8-position transferred thereinto.