The process planning expert system is discussed in this paper. The parts and structure to be constructed are expressed in terms of the Entity-Relationship model. The expert system has three particulars. At first, the production system is based on the modified Role method which was developed on the previous work of authors. Secondly, this system can recognize the substructure in the whole structure. The substructure is also process planned and treated as an assembly in the process planning of the whole structure. Thirdly, the process planning expert system consists of sub expert systems. Each sub system is in charge of the production at each stage, such as the Role generation, substructure recognition, process planning of the whole structure and so forth. With using this system, the system can save execution time and it can be made very clearly. The system was applied to the simplified structure of one block near the midship. The system proved its ability to give an appropriate process planning.