Abstract: For evaluating decision conditions, by the MDL principle, used for discriminating a given amino acid sequence as a particular one of super families, a sum of the description lengths of each of the decision conditions and a precision of each decision condition is calculated to make an evaluating parameter. Each decision condition comprises a plurality of conditional clauses. A description length of the decision condition is given by a total of clause description lengths of the conditional clauses. One of the clause description lengths is given by a total of a description length of the particular super family and a description length of one or more sequence motifs and a distance between motifs contained in the clause. An optimum is determined as one of the decision condition which makes the minimum evaluating parameter.