This book highlights the roles of inter-regional networks in regional economies to explore the drivers of sustained regional economic growth. Many industrialized countries are currently undergoing a period of population decline. To enhance sustainability in the regional economy, it is necessary to increase productivity and improve energy efficiency. This book provides new approaches to describing the economic effects of inter-regional networks, which are key to enhancing regional economic growth, using productivity analysis. In addition, it also furnishes considerable evidence on the formation of high-speed transportation infrastructure. Traditional studies on agglomeration economies have focused on external economies that occur in spatially limited areas and have not considered agglomeration economies from a broader perspective, i.e., from the perspective of inter-regional networks. In particular, recent studies have identified that the actual spatial range that would benefit from agglomeration economies is broader than that covered by conventional studies. This volume explains the phenomenon using Alonso's concept of borrowed size. This is the first book to show the impact of inter-regional networks on Japan's regional economy using the concept of the borrowed-size effect. Based on empirical evidence, the roles of inter-regional networks are determined in the context of the regional economy that faces population decline and environmental constraints. A deeper consideration of the analytical methods and understanding the results of detailed analyses will make it possible to propose desirable regional economic policies in the face of population decline. This book thus provides valuable insights into the regional economic development of Japan, which is particularly pertinent to other countries with similar land structures.