A market-demand-forecasting-value calculation section acquires order entry actual achievement data from an order-entry-actual-achievement-data storing unit of an order receiving system, and calculates a market demand forecasting value using the order entry actual achievement data. A correction safety inventory quantity calculation section calculates an error rate, which becomes negative when an ordering required-quantity becomes large, based on a number of ordered parts and a past ordering required-quantity. Subsequently, the correction safety inventory quantity calculation section calculates a correction safety inventory quantity using the error rate and the market demand forecasting value and a money-amount-zone index value. Further, the correction safety inventory quantity calculation section calculates a determined safety inventory quantity which is a value obtained by adding the correction safety inventory quantity and a reference safety inventory quantity calculated based on the market demand forecasting value.