Parallel isolation of a double-stranded nucleic acid and a single-stranded nucleic acid is possible from a sample that contains these acids, without separating the acids, by mixing the sample with a lysis buffer having high salt concentration or low salt concentration, or having a proteolytic enzyme. The sample that contains nucleic acid before its lysis, or the sample that has already been lysed or homogenized, is adjusted with a binding buffer in such a manner that the total nucleic acid is adsorbed onto a solid carrier. The binding buffer contains at least one non-ionic detergent in a high concentration. With the exception of the detergent, the sample contains no other non-acidic organic component miscible in water. The carrier with the adsorbed total nucleic acid is removed. The adsorbed total nucleic acid is washed and eluted.