This study, based on the new justice paradigm of Cropanzano, Byrne, Bobocel, and Rupp (2001), distinguishes justice into event justice and social entity justice, and investigates the relationships among event justice, social entity justice, and organizational member attitudes. To this end, organizational justice perceived by acquired firm employees during the post-M&A integration management process was divided into event justice regarding a series of M&A-related events and social entity justice toward the acquiring firm that planned and led the M&A. The study analyzed the effects of M&A event justice perceived by acquired firm employees on their attitudes through social entity justice toward the acquiring firm. For the empirical analysis, a survey was conducted among employees of acquired firms that were in the post-M&A integration management process following official M&A announcements among recently completed M&As, where acquiring and acquired firms were clearly distinguishable (N=192). The analysis results showed that distributive event justice, procedural event justice, and interactional event justice all had a significant positive (+) effect on social entity justice toward the acquiring firm. Additionally, social entity justice had a significant positive (+) effect on commitment to change and a significant negative (−) effect on turnover intention. Furthermore, to analyze the mediating effect of social entity justice on the relationships between event justice and commitment to change as well as between event justice and turnover intention, alternative models were proposed and comparatively analyzed, with an additional Sobel test conducted. The analysis results indicated that the mediating effect of social entity justice on the relationships between event justice and commitment to change, and between event justice and turnover intention, was significant. Based on these research findings, the significance, limitations, and future research agenda of the study were discussed.