The purpose of this paper is to present a network analysis of Japanese keiretsu groups. Inter-firm relations of six major Japanese keiretsu groups are analyzed from the social network perspective and following conclusions are drawn. First, keiretsu is an inter-firm network in which inter-firm transaction ties connect different industries. Second, keiretsu ties are mapped on the specific parts of inter-industry network where heavy market constraints are posed by the domestic markets. Third, the form of keiretsu should be considered as an intermediate trganiuation between marpet and firm. Hypothesis that keiretsz ties are mapped on dyadic transactions where industrial network poses heavy market constraints is statistically supported.