The production of semiconductor devices manufactured through a plurality of manufacturing sites is unitarily managed and an appropriate production plan is instructed. A computer projects a production plan of an entire company based on various information. The production plan is provided to each manufacturing site as a production instruction and provided to each business person or customer as a storing reply or an order accepting period reply. If the projected production plan (possible production volume) does not coincide with a production plan (necessary production volume), parameters obtained by correcting production allocation, production capability, lead time, yield and the like are re-input from a parameter input terminal. Based on the corrected parameters, the computer re-projects the production plan and projects an optimum production plan.