Abstract “Monthly Planning System of Production Activities of Kashima Steel Works” has been developed and is contributing greatly to the operation of the steelworks and to the reduction of the production cost. This system consists of four subsystems, i.e., the maintenance planning system, the production planning system, the energy planning system, and the data linkage system, the former three supporting the corresponding planning, and the last administering the communication among them. The main features of this system are (1) separation of functions to each sub-system, (2) on-line real time processing, (3) optimization, (4) man-machine dialogue-type processing, and (5) file arrangement suitable for case studies.