PURPOSE: To determine base sequence at a high accuracy, by applying a specified signal processing to a digital signal obtained by optically reading out an autoradiograph to correct distortion and deviation of the autoradiograph.

CONSTITUTION: Autoradiographs of separate development trains 2, 3, 5 and 6 of respective base-specified DNA synthetic matters of guanine, adenine, thymine and cytosine imparted a radioactive label and separate development trains 1, 4 and 7 of reference mixtures containing all of these synthetic matters are recorded on a photosensitive material and then, optically resd out to be converted into a digital signal. As for the signal, first, sampling points of the trains 1W8 are detected. Then, continuous lines L1WL4 are set to connect sampling points of the trains 1, 4 and 7 as reference train. Then, the continuous lines L1WL4 are compared with the trains 2, 3, 5 and 6 to identify the sampling points of the trains 2, 3, 5 and 6.