In order to develop social governance to manage natural resources, all actors around the commons have to make a consensus on who should be approved rights to manage them, and on what are reasons or values to prove the rights. This article defined legitimacy as approachability of others’ or one’s own rights to manage commons and discussed on theoretical or practical means for investigating the legitimacy based on results of surveys in Okinawa prefecture, Inner Mongolia autonomous region, and experiments using a simulation gaming. The surveys in Okinawa suggested that consistency of judgments to legitimacy among actors put governance managing coral reef into effect appropriately, and also revealed that institutional substance like legality interfered on perceived substance such as trustworthiness to judge legitimacy among people. Another research in Inner Mongolia indicated inconsistency of legitimacy among actors in accordance with their different evaluation of grass-land. Moreover, results of a simulation gaming “Who & Why Game” revealed that deliberate among actors shifted their mutual approval structure of legitimacy. Importance of frame of legitimacy on managing of commons, or making public policies was discussed.