The purpose of this study is to establish a geographic information systems (GIS)-based specialized information system using statistical information on both population and public libraries to support public library building projects. This developed system, called GIS-based Library Planning Support System (GLPSS), consists of a database and three operation modules which provide specific outputs from the data. The database includes geographic information, statistics on the population, and library information. One of the operation modules generates a service population using both grid technique and population density of the service area. Another operation module, a physical space program for a public library, is created with reference to an analysis of space in 141 public libraries. The third operation module is designed to calculate building costs and is based on specialized guidelines for library buildings. GLPSS provides for the building of public libraries in optimal regions, reflecting the characteristics of the local society and the distribution of the service target population. Because of its web-based nature, this support system has the advantage of convenient information updating and management. If GLPSS is applied to the planning of public libraries, it will also be able to support the planning of building projects by other authorities by providing similar public library building cases according to the regions and prevailing circumstances.