One of the problems in supply chain management (SCM) is the bullwhip effect, a phenomenon that amplifies fluctuations in the quantity purchased or ordered. Fluctuations in the quantity ordered by customers are small, fluctuations in the quantity ordered by retailers to wholesalers are large, and fluctuations in the quantity ordered by wholesalers to manufacturers are even larger. The amplification of demand fluctuation makes supply-demand management and production management difficult, making it an important issue for copier manufacturers and their component manufacturers located upstream in the supply. The bullwhip effect is supposedly affected by factors such as inaccurate demand forecasting, long production and logistics lead times, and minimum order quantities. This study developed a demand forecasting method for production control by clarifying and improving the factors that make it difficult to accurately forecast demand for products sold by copier manufacturers. Furthermore, we developed a demand forecasting method based on a voting system using a collective intelligence mechanism. The proposed method was devised to overcome problems such as the “beauty voting” problem in existing forecasting methods (prediction markets) using collective intelligence. A comparison of six months of data during the study period showed the amount of inventory by approximately 80% and reduced the shortages to zero for the product under consideration.