1. A method for rendering a three-dimensional space which includes an object with a mirror, comprising the steps of: segmenting the surface of said object into a plurality of polygonal elements; generating a polyhedron which includes a predetermined point in said three-dimensional space in the interior thereof, performing a rendering process for each surface of said polyhedron with said predetermined point as a view point, and storing rendered image; calculating a reflection vector at each vertex of said polygonal elements from a view point used when the entire three-dimensional space is rendered; identifying a surface of said polyhedron in which an intersecting point between said reflection vector with said predetermined point as a start point and said polyhedron exists, checking whether the surfaces where said intersecting point exists are all identical for each vertex of one polygonal element; and if all of said surfaces are identical, calculating a coordinate in said rendered image which corresponds to each vertex of said polygonal elements by using said surface where said intersecting point exists and said reflection vector; and if all of said surfaces are not identical, clipping said polyhedron by a polygonal pyramid whose apex is said predetermined point and whose ridgeline is said reflection vector of each vertex of said one polygonal element, and calculating a reflection vector on a boundary line of each surface of said polyhedron; calculating a coordinate in said rendered image, which corresponds to said reflection vector on said boundary line, from said reflection vector on said boundary line; and calculating a point on said object which corresponds to said reflection vector on said boundary line; and performing a texture-mapping of said rendered image onto said surface of said object by using said coordinate in said rendered image which corresponds to each vertex of said polygonal elements and a calculated point; and displaying a result of the texture mapping.