PURPOSE: To simplify extraction of nucleic acid, removal of viral nucleic acid, etc., and provide high yield in the field, etc., of genetic engineering by bringing a nucleic acid-containing sample into contact with a protein modifying agent under alkaline conditions, then adding an alcohol thereto and precipitating the nucleic acid.

CONSTITUTION: An alkaline substance such as NaOH is added to a sample such as cell, bile or excreta converted into a state capable of contact with a protein modifying agent, etc., by crushing, stirring, etc., to provide pH ≥7 (preferably pH ≥8.0). The protein modifying agent such as guanidine hydrochloride, urea or sarkosyl in a required amount is then added and the obtained mixture is stirred. An alcohol such as n-propanol is further added thereto precipitate nucleic acid and carry out centrifugation, etc. Thereby, the objective nucleic acid is extracted or removed.