The number of times of manifestation of each of a plurality of partial base sequences obtained from own base sequence data containing a target base sequence is counted, and held as an own frequency table. The number of times of manifestation of each of the plurality of partial base sequences is also counted for competing base sequence data to be distinguished from the own base sequence data, and held as a competing frequency table. In a probe evaluation step, the frequency information in the own and competing frequency tables is displayed so as to be comparable with reference to the partial base sequences, and at least one of the plurality of partial base sequences is determined according to instruction operations made by a user, thereby forming probe candidates based on the determined partial base sequences.