The present invention pertains to: a method for modifying a nucleic acid contained in a sample, the method including a step for bringing the sample into contact with a nucleic acid-modifying agent in the presence of an acidic polysaccharide and/or a nucleotide; and a method for selectively detecting a nucleic acid derived from living cells contained in the sample, the method including the following steps: (a) a step for modifying a nucleic acid contained in a sample according to the method for modifying a nucleic acid contained in a sample, which includes a step for bringing the sample into contact with a nucleic acid-modifying agent in the presence of an acidic polysaccharide and/or a nucleotide; and (b) a step for selectively detecting an unmodified nucleic acid from the sample after step (a). The present invention further pertains to a kit and composition for use in these methods.