Based on a case study of Borderless Japan Co., Ltd., this study aims to illustrate the institutionalisation process of a management accounting system—from the emergence to the formalisation of social impact in developing social businesses. More specifically, by applying institutional entrepreneur theory, we analyse how the company institutionalised social impact as the formal system to reproduce the founders’ experiences. The case analysis shows that the reflexivity of the institutional entrepreneurs, who are embedded in the institutional contradiction between the social and economic logics inherent in social business, is significant in the institutionalisation. Furthermore, an important process for this institutionalisation is the theorisation of social impact as an organizational normative rule. This influences new social entrepreneurs and their businesses to maintain the fundamental principle of social business: the pursuit of social impact as an end and economic profit as a means.