An object of the present invention is to provide a method for separating and purifying a nucleic acid by adsorbing the nucleic acid in a test sample to a surface of a solid phase and desorbing the nucleic acid by washing and the like. The present invention provides a method for separating and purifying a nucleic acid having a predetermined length from a nucleic acid mixture, comprising a step of: adsorbing and desorbing a nucleic acid in the nucleic acid mixture containing nucleic acids having different lengths to and from a solid phase of an organic macromolecule having a hydroxyl group on surface thereof.