To provide an optimization system for production plan, which is capable of optimizing the production plan, regardless of the presence or absence of empirical knowledge.

An optimization system for production plan is a system for calculating an optimum production plan in consideration of the productive efficiency based on the dimensions and the production quantity of automotive mufflers. Based on the scheduling knowledge base 1 accumulating a collection of knowledge of production plan empirically obtained from the relation between the dimensions, the production quantity of the mufflers, and the productive efficiency, and the inputted dimensions and production quantity of the mufflers, knowledge to be applied is retrieved from the collection of knowledge accumulated in the scheduling knowledge base 1, and then an inference engine 2 which infers based on the knowledge, is provided.