An inventory management method or system for controlling amounts of stocks is provided. The method comprises the steps of: storing delivery times and quantities of order lots accepted and restock lead times predetermined every item in a storage; selecting at least one order lot accepted, having a delivery time within a period from "b" days before to "a" days after a basic time, from said order lots accepted, to set a threshold value x+y.sigma..sub.1 (where y.gtoreq.0) based upon both an average x and a standard deviation .sigma..sub.1 of quantities of said selected order lot accepted; classifying at least one order lot accepted having a quantity .alpha. of the selected order lot accepted as a normal accepted order lot, wherein the .alpha. is less than or equal to said threshold value and classifying at least one order lot accepted having a quantity .alpha. of the selected order lot accepted as a special accepted order lot, wherein the .alpha. is greater than said threshold value; and calculating a basic stock using a predetermined formula from said classified normal order lot accepted.