The present study defined legitimacy as approvability of others' or one's own rights to participate in managing commons and proposed two kind of determinants, institutional substance as a reference frame based on such matters as regal norms and perceived substance founded on a subjective estimation of others' or one's own desirability and the like. When an actor's rights to manage commons are established by legality as institutional substance, people will recognize their rights to be structured as a low variability. Therefore, their cognitive process for considering the grounds of the actor's rights besides legality will be disturbed. In this case, it is hypothesized that it becomes difficult for perceived substance such as trustworthiness to act as a determinant of legitimacy as a result of inhibition to attention. To examine the interference effects of institutional substance on perceived substance, a research survey was conducted to measure the evaluations of actors who participate in making policies for the prevention of red-clay flow at Onna village in the islands of Okinawa. The results indicated that the trustworthiness of actors promoted legitimacy when their legality was evaluated as low. We offer a theoretical discussion on the legitimacy of rights and its determinants around the management of commons.