要旨：This paper examines a conventional assumption of transport costs; any bilateral transport costs are symmetric. Using a comprehensive dataset on transport transactions in Japan, we first describe a directional imbalance of transport costs by transport mode and examine its potential sources such as density economies, long-haul economies, and a directional imbalance of transport flow. Based on the method of instrumental variables, we find that the density economies drive a deviation from symmetric transport costs between prefectures. We then develop a theoretical model of economic geography in which asymmetric transport costs occur in a transport sector for the density economies. Compared with the model without density economies, we show that agglomeration of economic activities is likely to emerge under any ranges of cost parameters. Thus, the economies of density in transport flows act as an agglomeration force.