An order amount of parts can be calculated from an actually used amount of the parts, so that the used parts can be resupplied subsequently. An order amount can also be calculated form a production plan, so that the parts necessary to meet the plan can be delivered.In this technique, an order amount is calculated by the equation "W1.multidot.X1+W2.multidot.X2", where X1 is the order amount calculated in a subsequently resupplying way, X2 is the order amount calculated in a plan correspondence way, W1 is an actual weight, and W2 is a plan weight. The actual weight W1 becomes high and the plan weight W2 becomes low when the subsequently resupplying way is suitable, and vice versa. The actual weight W1 and the plan weight W2 can be calculated from experience or system analysis.