PURPOSE: To correct deviations or bendings caused by incomplete electrophoresis by placing control samples at both sides of a DNA fragment sample by a base- specific breakage of DNA.

CONSTITUTION: DNA is broken base-specifically to fragment samples of T (thymine), C (cytosine), G (guanine), and A (adenine). The control samples which are prepared by mixing the above fragmented samples, are placed between the above fragmented samples. Then an electroporesis is conducted. Due to some incompleteness of the electrophoresis zones deviate upward to left on the left side, and upward to right on the right side. Then, mean values, -YTn=(Y'n+y"n)/2, are calculated by measuring mining the positions, y'n and y"n, of respective control samples, 1 and 2, in the n-th zone. Then, the position of the DNA fragment of T is determined by judging whether the zone of T line is present at the position -yTn. As to other samples regarding to C, G and A, procedures similar to the above are carried out to determine positions of DNA fragments.