Adenylation of a DNA fragment with a DNA polymerase occurs in the course of PCR, and thus two peaks are detected. To prevent the peak splitting, it is necessary to raise efficiency of adenylation a single peak to occur without changing reaction conditions. To this end, four types of PCR primers which, respectively, have an anchor sequence at 5′ terminus so that any of A, C, G or T is attached to at the 5′ terminus of the anchor sequence, and PCR is carried out by use of the respective primers to determine efficiencies of adenylation. Subsequently, an anchor sequence that is more likely to undergo adenylation is screened to decide an anchor sequence more likely undergo adenylation, followed by PCR by use of a primer having the decided anchor sequence.