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; 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; editing the decision tree generated; generating the constraint condition from a modification history of the decision tree edited; and storing the constraint condition in the external storage device, wherein the constraint condition is composed of at least a first constraint condition stipulating an ignored attribute, a second constraint condition stipulating that an attribute is subordinate to other attributes, and a third constraint condition stipulating that an attribute is superordinate to other attributes, and the generating the constraint condition comprises generating the first constraint condition that ignores an attribute if the attribute is deleted from the decision tree, generating the second constraint condition stipulating that an attribute is subordinate to attributes other than the attribute if the attribute is moved to a position further from a root of the decision tree, and generating the third constraint condition stipulating that an attribute is superordinate to attributes other than the attribute if the attribute is added to the decision tree or if the attribute is moved to a position closer to the root of the decision tree.