Abstract: A rendering method is provided that can materialize rendering processing from volume data at high speed using a surface rendering processor without restrictions on the structure of volume data to be processed. In the method, the sampling points of volume data V over a 3-D region are, in principle, arranged on the surfaces Q.sub.1, Q.sub.2, . . . Q.sub.n in equal intervals along a viewing ray L around viewing point p, and these surfaces are extracted as a set of triangles T.sub.i. Then, the volume data at each vertex of these triangles and the gradient vectors of this data are obtained by interpolation and generated as a triangle data group. High-speed volume rendering is materialized by displaying the data groups of these partially transparent triangles using a surface rendering processor. Since existing programs can be used for generation and processing of triangle data groups, there are no restrictions on the structure of the volume data to be handled.