A signal processing method in autoradiography for determination of the base sequence of DNA or a DNA fragment, employing groups of radioactively labeled base specific cleavage products or mixtures thereof obtained by specifically cleaving the DNA or DNA fragment labeled with a radioactive element and resolved one-dimensionally in parallel relation to each other to form resolved rows on a support medium. An autograph is obtained having locational information on groups of radioactively labeled cleavage products contained in the rows on the support medium. An electrical digital signal corresponding to the autoradiograph are generated by recording and reproducing a radiation image stored in a stimulable phosphor sheet. Sampling points in each resolved row of the digital signal are next detected. Reference sampling points are then determined in a plurality of reference rows which are either directly provided on the support medium and/or synthesized from certain resolved rows. Corresponding reference sampling points in the plural reference rows are joined to generate plural continuous lines comprising straight lines, polygonal lines or curved lines. Sampling points in the remaining non-reference rows are identified by comparison with the continuous lines to thus determine the base sequence of the DNA or DNA fragment.