This study investigates how public libraries serve people with developmental disabilities through in-depth interviews with librarians experienced in planning and operating library services for the disabled to find out what requirements are for public libraries to successful serve people with developmental disabilities in the future. For this purpose, eight librarians working in the public libraries of Seoul’s three autonomous regions were interviewed individually, and the interviews were analyzed. From the interviews, two themes, nine sub-themes, and 39 content elements emerged. The librarians who participated in the interview recognized the necessity of the service for the disabled and had tried various types of services. Even in difficult circumstances, they tried to successfully implement the service by applying their own execution strategy, and as a result, they were able to bring about changes in user satisfaction, positive response, and changes in the perception and attitude of others. The librarians also explained the difficulties they faced during their service. They said that it is necessary to develop the capacity of librarians and other library staff, improve user awareness, make policy changes and try cooperative approaches. Based on these research results, the significance of the points found from the librarian s experience was considered, and the discussions focused on the requirements necessary for public libraries to change into ‘libraries for all’ regardless of disability as social public institutions.