Abstract: A required inventory calculating system includes a first section for calculating product importance level coefficients, a second section for calculating procurement risk level coefficients, a third section for calculating general coefficients by multiplying the product importance level coefficients by the procurement risk level coefficients, a fourth section for calculating required inventory periods respectively with respect to raw materials or partly finished products by multiplying the general coefficients by a predetermined longest inventory period, and, if the required inventory periods are shorter than a predetermined shortest inventory period, setting the required inventory periods as the shortest inventory period, and a fifth section for calculating required inventories respectively for the raw materials or the partly finished products by multiplying required quantities of the raw materials or the partly finished products required to supply the products to customers or wholesalers in a predetermined p