In existing literature on diversification strategy, the view has been presented that the level of decentralization between corporate headquarters and affiliates is determined by the business unit's competitive strategy. Contrary to this theory, this paper proposes and empirically analyzes the logic that what determines the level of decentralization between corporate headquarters and business units in diversified firms is not the "necessity" for decentralization based on the business unit's competitive strategy, but rather the business unit's "bargaining power" over corporate headquarters derived from the business unit's possession of resources. To this end, this paper analyzes the influence of two factors—the level of resource possession and the exploitability of resources held by business units—on the determination of the decentralization level. Additionally, it empirically reveals how the corporate headquarters' position on decentralization changes when these two characteristics of resource possession level and exploitability interact. Furthermore, this paper distinguishes the concept of decentralization—which prior research on decentralization had not differentiated—into two types: decentralization regarding operational matters and decentralization regarding strategic matters, and examined which type of decision-making matters actually involve delegation of authority between corporate headquarters and business units. The hypothesis testing results yielded the following conclusions. First, it was confirmed that among the various decisions faced by headquarters and business units in diversified firms, decision-making authority regarding operational matters is already granted to business units, and the adjustment of decentralization levels between headquarters and business units is limited to cases where the decision-making type concerns strategic matters. Second, while the resource possession level and resource exploitability of business units can serve as sources of bargaining power over corporate headquarters, thereby raising the level of decentralization regarding strategic matters, it was found that when resource exploitability is high and a business unit is judged to possess abundant resources, a tendency toward centralization actually emerges. This paper has significance not only in attempting a conceptual distinction among various decision-making matters related to decentralization within diversified firms, but also in additionally confirming, as part of research identifying factors that influence the determination of decentralization levels between corporate headquarters and business units in diversified firms, the influence of resource exploitability as a characteristic beyond the resource possession level that appeared in existing research, based on a resource dependence perspective.