Studies on synergy implementation argue that corporate headquarters must directly intervene in the decision-making of lower-level business units by increasing the degree of centralization to create synergy. However, it is questionable whether relying solely on the single mechanism of centralization can effectively address all types of synergy creation situations. This is related to the fact that existing diversification studies have focused on the measurement of relatedness indices while overlooking the possibility that conditions for synergy creation beyond relatedness may exist. In other words, because synergy research has centered on inter-business unit relatedness, discussions on implementation have inevitably been limited. Had diverse synergy creation situations been reflected, diverse means for synergy creation would likewise have been considered. Synergy cannot be created solely through centralization by corporate headquarters, and inter-business unit relatedness is not the only condition for synergy creation. This paper presents four situational conditions for synergy creation—inter-affiliate relatedness, affiliate competitive strategy, affiliate synergy creation capability, and affiliate synergy creation willingness—and proposes centralization and subjective performance evaluation as modes of corporate headquarters intervention for synergy creation. It argues that depending on the synergy creation situation, one of four intervention modes is selectively employed: centralization, centralization combined with subjective performance evaluation, decentralization, or decentralization combined with subjective performance evaluation. According to the empirical analysis results, when inter-affiliate relatedness is high, headquarters intervenes through centralization combined with subjective performance evaluation; when affiliates possess superior synergy creation capability, through decentralization combined with subjective performance evaluation; and when affiliates exhibit strong synergy creation willingness, through decentralization. However, when affiliates' competitive strategy is oriented toward cost leadership, no statistically significant results were obtained, calling for further research on this matter. The significance of this paper lies in its proposal and empirical demonstration that corporate headquarters need not rely on centralization as the sole mechanism for synergy creation, but can intervene in affiliates through various combinations of centralization and subjective performance evaluation, with the most effective combination being determined by the synergy creation situation facing the affiliates.