A constraint condition DB storing a constraint condition that stipulates a structure of a decision tree is referenced, and a decision tree is generated from a case set where values of a plurality of attributes and a conclusion are associated with one another so that the structure of the decision tree, which is stipulated by the constraint condition, is satisfied. Accordingly, for example, even if a new case is added to the case set, a basic structure of the decision tree is succeeded by the constraint condition, thereby avoiding a situation where an operator needs to significantly modify the decision tree. Therefore, operations of modifying the decision tree by the operator can be reduced.