Abstract: An expert system which has a plurality of knowledge modules 2 for expressing knowledge of a human expert and having a function for managing the knowledge. The knowledge base modules 2 include a plurality of knowledge respectively and a certainty factor is determined for the knowledge respectively. There is a knowledge base compiler 3 for managing the knowledge base modules 2, extracting a plurality of necessary knowledge base modules from a plurality of the knowledge base modules 2 during execution of an inference process and synthesizing synthesis knowledge base as one knowledge base by synthesizing the extracted knowledge base modules. There is a CF manager 4 for matching a certainty factor determined to the extracted knowledge base modules respectively by the knowledge base compiler 3.