1. A non-transitory computer readable medium storing decision tree generating program for causing a computer having an external storage device storing a constraint condition that stipulates a structure of a decision tree to execute a procedure, the procedure comprising: reading a case set where values of a plurality of attributes and a conclusion are associated with one another; reading the constraint condition from the external storage device; and generating a decision tree from the case set read so that the structure of the decision tree, which is stipulated by the constraint condition, is satisfied.