In this paper, a prototype of Operator Support System is developed to support operators in abnormal plant condition. The system has following two functions; one is to assist operators for decision-making in case of recovery operation equivalent to expert operators. Another is to prevent operation errors by monitoring operator?s action. In recent years, Japanese industry faces serious problem that skilled operators are reaching their retirement age. It is a great concern that the operation skills such as decision-making would begin to decline after the retirement of skilled operators. In addition, their accumulated experience and knowledge, which cannot be sufficiently explained in manuals alone, will be lost from plant workplaces. The Operator Support System proposed in this paper eliminates such concerns.