We have developed a new expert system for assessing the applicability of TBM tunneling in Japan. We initially planned to construct the expert system on the basis of a unified knowledge base or set of rules without contradictions. However, it was found after several attempts that it is very difficult to unify knowledge in this field because of the wide range of opinions and models and the continuous development of TBM technology. As a result, the system was divided into three stages. In stage A, the fulfillment of fundamental requirements is assessed. Stage B is a simple expert system consisting of minimal set of suitable rules based on the author's research. Stage C incorporates a wide range of models suggested by different experts and the over-simplified and omitted points in stage B. Although the accuracy of the output of this expert system can certainly be improved, the method for accumulating knowledge and rules makes this system is simple and easy to use. We plan to continue development of this system according to this principle.