3D city models are important in urban planning for sustainable development. Urban planners draw maps for efficient land use and a compact city. 3D city models based on these maps are quite effective in understanding what, if this alternative plan is realized, the image of a sustainable city will be. However, enormous time and labour has to be consumed to create these 3D models, using 3D modelling software such as 3ds Max or SketchUp. In order to automate the laborious steps, a GIS and CG integrated system that automatically generates 3D building models is proposed based on building polygons (building footprints) on a digital map. In either orthogonal or non orthogonal building polygons, the new system is proposed for automatically generating 3D building models with general shaped roofs by straight skeleton computation.