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 is then generated. 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 resolved row adjacent to the reference row to identify sampling points in adjacent resolved rows. Sampling points in the remaining non-reference rows are identified for determination of the base sequence of the DNA or DNA fragment.