A DNA fragment preparation method for DNA analysis comprising, i) preparing a plurality of DNA fragments from a sample DNA, and ii) amplifying a specific DNA fragment by PCR, using a pair of primers which hybridize with terminus sequences of the DNA fragments, and a specific primer which hybridizes specifically with a base sequence of the specific DNA fragment at a position between a priming site of one of primer the pair of primers and a priming site of another primer of the pair of primers. The specific primer hybridizes specifically with a base sequence at a middle position of the specific DNA fragment. Products of PCR are separated, by electrophoresis, and signals from DNA fragments originated in a known genes and signals from DNA fragments originated in a unknown genes are displayed separately on a display.