A general-purpose optimal operational planning system with a user-friendly interface is developed for the purpose of determining operational strategies of energy supply plants in a simple and rational manner. The system has the following main functions: (a) data registration, (b) graphical flowsheet editing, (c) automatic programming and optimization calculation, and (d) graphical representation of results. A graphical flowsheet editor enables an easy and flexible composition or change of plant configurations and their relevant data by using fundamental data registered in a database. Plant data composed by the editor are arranged automatically to carry out an optimization calculation, in which the operational strategy is determined so as to minimize an objective function, such as operational cost subject to energy demand requirements and other operational restraints. An application example shows that the system is a useful tool for the operational planning of energy supply plants from the viewpoints, not only of rationality, but also of task and lime reduction.