The purpose of the present invention is to provide a substance having a several ten to several hundred-fold exchange reaction accelerating activity as compared with that of conventional copolymers. In particular, the invention provides a preparation for accelerating an exchange reaction between a nucleotide sequence at specific site of a double stranded DNA or RNA for its homologous nucleotide sequence, the preparation comprising a cationic polymer having a guanidine group-containing main chain and a hydrophilic functional groups as an active ingredient. Thus, a substance having a several ten to several hundred-fold exchange reaction accelerating activity as compared with that of conventional copolymers can be provided. With this substance, the nucleotide chain exchange can be performed at a lower temperature and/or a higher rate than in the prior art.