Cluster policy is designed to facilitate inter-firm networking. We examine industrial clusters in Japan based on firm-level transaction data. Firms in clusters expand transaction networks at a higher speed, but significantly only with firms in the agglomerated core Tokyo, not with local firms within the same region. We confirm the robustness by regional historical background as instruments. By disaggregating firms by their main bank types, we find that cluster firms expanding networks are mainly financed by regional banks, not by banks with nation-wide operations. This suggests the importance of intensive relationship with the main banks for inter-firm network formation.