This chapter examines how interregional accessibility influences the efficiency of Japan’s regional innovation system (RIS), focusing on patent intensity efficiency. Previous studies provide limited insight into the conditions under which Borrowed Size, facilitated by high-quality high-speed rail systems, promotes innovation activities and enhances RIS functionality. This chapter addresses this gap by evaluating the determinants of RIS efficiency through stochastic frontier analysis using patent data. The results indicate that employment density and establishment size significantly affect patent intensity, confirming the role of agglomeration economies. Regional innovation activities are concentrated in large-scale establishments, underscoring the importance of substantial research and development investment. Moreover, the Borrowed Size effect emerges as a significant determinant of RIS efficiency, with accessibility to large metropolitan areas enhancing regional innovation capacity. Conversely, the implementation of high-speed transportation systems, such as the Chuo Shinkansen (Maglev), may exacerbate economic disparities between large metropolitan areas and geographically isolated rural regions.