PROBLEM TO BE SOLVED: To provide a preparation method for a specimen by which a cell with an appearing small nucleus can be discriminated clearly and to provide a preparation method for a specimen by which a juvenile red corpuscle and a mature red corpusle can be discriminated easily, precisely and with high sensitivity. SOLUTION: In the method, a cell specimen which is used to monitor the damage of a nucleic acid is prepared. The method is provided with a step in which the specimen is stained with a staining solution whose acridine orange is at 10 to 40μg/mL.