Consistency among actors’ evaluation of who has the right to decide on public policies that affect the interests of various people is an important premise for social governance. The present research defined legitimacy as the approvability of rights of others and the self, to participate in the management of commons, and distinguish between institutional legitimacy and perceived legitimacy. The former is secured by institutions like the legal system, whereas the latter is based on individuals’ subjective estimation like trust without institutional assurances. An interview survey targeted three actors, civil servants, a member of the local fishery cooperative, and another local resident on the red-clay runoff problem regarding damage to the shoreline and sea in Onna village in Okinawa. As a result, civil servants approved of the institutional legitimacy of the fishery cooperative, and a local resident approved of its perceived legitimacy on the basis of trustworthiness. Theoretical and applicable contributions of two types of legitimacy to expand social governance in managing commons are discussed.