When the nucleic-acid base sequence of A, C, G, and T (or U) is determined by interpreting fluorescent-light intensity waveform data acquired by measuring nucleic-acid fragments, it is desirable to determine, with a high-accuracy, the base sequence at a location at which the data interpretation is difficult. In order to accomplish this object, the data interpretation is performed by making reference to information acquired by performing the statistical processing to plural pieces of fluorescent-light intensity waveform data corresponding to already-known base sequences. This method allows the determination of the nucleic-acid base sequence at the above-described location.