1. A three-dimensional model generating system for generating a building model on the basis of map information describing the arrangement of building regions and roads on a two-dimensional plane and elevation-data that is generated by mapping altitude information to each observation point on the two-dimensional plane, comprising: a coordinate transform obtaining unit that performs coordinate transformation from the map information to the elevation-data; a building region extracting unit that arranges the altitude information according to each building region; a plane extraction unit that extracts planes including the altitude information on the basis of the altitude information according to each building region; a boundary line identifying unit that identifies a boundary between the planes; and a plane connection unit that connects the planes, between which the boundary has been identified, whereby the building model formed of a simply-connected polyhedron is generated.