The accuracy of forecasting is guaranteed for applying the time related forecasting method under normal circumstances. However, man-made situations, such as planned order can lead to completely different forecasting results. The research to enhance the accuracy of forecasting is meaningful in that it can be improved by controlling the details of planned order. In this study, an effect of planned order is introduced and the method to enhance the accuracy of demand forecasting in the planned order system is proposed. The validity of the system is examined by comparing the effectiveness of demand forecasting under the planned order system.