Farm mechanization in Japan has remarkably developed in last decade. On the other hand, various accidents occurred in using farm machinery. Farm work safety in association with farm mechanization has become increasingly necessary in recent years. In this paper, an expert system to prevent accidents on farm was built for farm work safety. This system was developed by using backward reasoning of an expert shell. Before developing the system, farm work accidents were analyzed, and the results were utilized as a knowledge base of the expert system. This expert system was evaluated by farm machinery specialists and proved to be useful as an educational system for farm work safety.