It is very difficult to exchange geographical data among geographical information systems which store their spatial data with independent storage structures. Since, moreover, large amount of storage space is necessary to store spatial data and ex pen sive cost is required to input them, waste will grow as they are stored redundantly. Therefore, it is essential to share them with other geographical information systems by exchanging spatial data among them. In order to exchange spatial data efficiently, there exist several international standards for data exchange format. In this paper, we design and implement a data conversion system that converts geographical data between SDTS (Spatial Data Transfer Standard) which is adopted as the national standard for common data exchange format and Informap which is the existing mapping system. We first analyze the storage structures of SDTS and Informap, respectively and develop gateway functions according to these analyses for efficient conversion. Finally, we design and implement the overall data conversion system between SDTS and Informap using the gateway functions.