There is provided a method of managing production quantity that includes determining a standard inventory quantity of products to be stocked in the single delivery center for a time of at least one predetermined cycle ahead based on a shipment record of the single delivery center, and determining a total assembly quantity of products to be assembled by the at least one factory for a time of at least one predetermined cycle ahead based on the determined standard inventory quantity of the single delivery center and an actual inventory quantity of the at least one factory.