1. A management apparatus managing a plurality of information processing devices, comprising: a working memory storing rule segments, each rule segment comprising a condition event and a conclusion, wherein each of the rule segments are divided into entities of rules, each rule comprising a combination of condition events to be generated in the information processing devices and a conclusion inferred by the combination of condition events; and a processor configured to: (1) receive an event notification corresponding to a content of an event transmitted from one of the information processing devices, when the event is generated in the one information processing device; (2) retrieve one or more of the rule segments related to the event notification and infer a certain conclusion by referencing the retrieved one or more rule segments; (3) update a corresponding portion of rule segment management information, wherein the rule segment management information includes a reference history for each of the rule segments; (4) select a certain one of the rule segments on the working memory which is not readily used, based on the rule segment management information; and (5) delete the certain rule segment from the working memory.