1. An expert system comprising: a plurality of knowledge base modules (2) for expressing a knowledge of a human expert and having a function for managing said knowledge, each of said knowledge base modules (2) including a plurality of knowledge, a corresponding plurality of certainty factors, a plurality of facts, and a plurality of production rules; a knowledge base compiler (3) for managing said knowledge base modules (2), extracting a plurality of necessary knowledge base modules from a plurality of said knowledge base modules (2) during execution of an inference process, and synthesizing a new unit of synthesis knowledge base by synthesizing said extracted knowledge base modules; a CF manager (4) for matching certainty factors of said extracted knowledge base modules, and for determining a uniform certainty factor of said synthesis knowledge base; an inference engine (5) for inputting data from external sources, and for executing an inference process based on said inputted data and said synthesis knowledge base, to obtain an inference result; wherein said CF manager (4) includes: means for inputting certainty factors of a plurality of said extracted knowledge base modules; overall tuning means for performing a tuning of said certainty factors corresponding to said knowledge base modules; means for calculating provisional certainty factors for said knowledge base modules tuned by the overall tuning means; individual tuning means for matching said provisional certainty factors among said knowledge base modules; and means for determining a uniform certainty factor in said synthesis knowledge base matched by said individual tuning means.