A rule base storing control rule sets simulates the behavior of each car of an elevator system in real time by assigning scanning to each car which is caused to run until the direction of running is reversed, while applying a specified rule set in the rule base to the current traffic condition, and predicts group supervisory control performance upon application of the specified rule set. In response to the results of performance prediction, an optimal rule set is selected and a real time simulation can be carried out during a group supervisory control operation, so that group supervisory control can be performed on multiple elevator cars while applying the optimal rule set at all times, thus providing excellent service.