Recently, some companies, such as Airbnb and Uber, have successfully provided services that utilize spare personal assets by sharing them with others. This idea, which is called the sharing economy, is predicted to have a major impact on society. In such a society, various devices will be shared with others. To share and utilize those devices efficiently, the permission level should be able to be controlled flexibly. Device owners generally do not want to share their devices with strangers, while they want to grant stronger permission to their close friends. However, none of the conventional authentication methods have provided such a functionality yet. Although device owners can manually configure the permission level for each user, it is a great burden. Therefore, this paper proposes a system that exploits online social relationships as a solution to the authentication problem when sharing devices. By acquiring and evaluating online social relationships between a device owner and a user, the proposed system automatically determines whether the user can access the device and how strong the permission for the user should be. The owner can make efficient use of the shared device without worrying about the access configuration of users. To examine the feasibility and effectiveness of the proposed system, this paper presents a prototype system and measures the performance of the prototype system. Furthermore, by simulating a specific scenario, this paper shows that the proposed system is capable of controlling the permission level for each user effectively based on the user's online social relationships.