A signal processing method for determining base sequence of nucleic acids by subjecting digital signals to signal processing, said digital signals corresponding to an autoradiograph of plural resolved rows which are formed by resolving a mixture of base-specific DNA fragments or base-specific RNA fragments labeled with a radioactive element in one-dimensional direction on a support medium,which comprises steps of:(1) detecting at least two bands continuously in the lower part of each resolved row and numbering the bands consecutively in order from the lower end;(2) obtaining correlation of a distance between the detected bands in the resolving direction with the band's number and predicting positions of undetected bands in the resolving direction from the correlation;(3) detecting at least one band on the resolved rows on the basis of the predicted positions and numbering the bands consecutively;(4) obtaining the correlation of the distance between the bands with the band's number for the already detected bands including the band newly detected in the step (3), and predicting positions of undetected bands from the correlation; and(5) repeating in order the steps (3) and (4) to thereby detect all bands on the resolved rows.