A safety stock amount calculation method includes calculating a probability Pb that a delivery time for a certain commodity required by a customer is shorter than its lead time L, calculating an average LL of a difference between the lead time L and the customer's required delivery time when the lead time L exceeds the customer's required delivery time, correcting an inventory adjustment period N by using the average value LL, and calculating a safety stock amount SS by the equations SS=Pb×k×(√N×F)×σ, wherein σ is a standard deviation of demand for the commodity, N is a corrected inventory adjustment period, Pb is a probability, F is a shipment frequency, and k is a safety coefficient.