The user-defined inference rule is created using a knowledge base search language that is basically provided in a knowledge base management system. Accordingly, the user-defined inference rule is a knowledge base search statement and is processed using a function of searching a knowledge base as a base function of the knowledge base management system. The user-defined inference rule is applied by including a knowledge base search process and a search result in the knowledge base, when the knowledge base is changed. The user-defined inference rule is managed by a user-defined inference rule manager. The user-defined inference rule manager monitors a change in the knowledge base to determine when the user-defined inference rule is applied, executes the user-defined inference rule to obtain the execution result, and includes the execution result in the knowledge base.