This study examines necessary and sufficient conditions in terms of legitimacy with respect to digital platform regulation in the context of today’s development and refinement of sophisticated science and technology. With the increasing oligopoly of platforms, the government has established data portability that can be shifted from a particular platform at any time, creating an environment in which new platform-based businesses can emerge and vigorous competition can take place. In terms of legitimacy, this chapter considers the following necessary conditions: first, sociological acceptance; second, moral legitimacy; and third, reliance on a foundation that is inherently uncertain but is allowable through expectations of the future. As a sufficient condition, the chapter also discusses applying social psychology’s elaboration likelihood model, which explains how persuasion is particularly effective in changing people’s attitudes. This approach analyzes “resources” and “reflectivity” in terms of motivating content processing, and “expertise,” “cognition,” and “consequences” in terms of judging the message based on non-essential peripheral cues. In addition, the point of causality is also examined, and, here, further empirical analysis is needed. The digital platform regulation that has been introduced in Japan has not been clarified in terms of necessity and sufficiency. In practice, these points must be met through policy implementation.