The present invention provides a method for obtaining or amplifying a polynucleotide (a tester-specific polynucleotide), in which an amount existing in a sample (tester) is larger than the amount existing in another sample (driver), easily and within a short time as well as with high efficiency, a polynucleotide obtained (amplified) by such method, a method for identifying gene mutation in the tester, and a kit to be used in such methods.