In a safety stock control system, safety stock is changed in response to updating of a setting value so as to be continuously maintained as a proper value. The safety stock control system is comprised of a demand prediction means, an actual value calculating means, a predicted remainder calculating means, a parameter calculating means, and a safety stock calculating means for calculating safety stock based upon a parameter calculated by the parameter calculating means. In response to updating of the setting value, data of an area, which is different from such a data read when a plan is made, is temporarily read from a data storage means to the respective means, and then, various sorts of calculating process operations are carried out either one time or plural times.