Abstract: A coprecipitant and a nucleic-acid extraction method using the coprecipitant are provided. The coprecipitant has affinity to the nucleic acids, no competitive inhibition to the reverse transcription and no inhibition to the PCR reaction in case of extracting a very small amount of nucleic acids by alcoholic precipitation using isopropyl alcohol or ethanol. Further, the coprecipitant can precipitate with the nucleic acids as a visible white or blue precipitate, thereby to suppress technical errors and enhance the extraction efficiency. A coprecipitant which acts in a process of extracting the nucleic acids by centrifugal separation from biological materials and/or test samples in the same manner as nucleic acids and has ability for precipitating as a visible white or blue precipitate when being separating and concentrating it by alcohol.