In the ECE regulation, any surface of the interior part of automobiles must have sufficient roundness where a sphere of diameter 165mm, which is equivalent to the average head size of infants, may collide. A system for automatically detecting sphere contacting shape on the automobile part is developed. In the current practice, some Japanese automobile manufactures detect the shape by using the virtual milling method. In this method, a milling simulation with a ball end cutter of diameter 165mm is executed on the part to detect the sphere contacting shape. The method generally needs a lot of computation time for the detection. In this paper, the authors propose an improved virtual milling method for fast detecting the sphere contacting shape. Our algorithm initially generates points sufficiently covering the visible surface of the model. For each point, the contact condition of the cutter is evaluated by using the inverted offset surface of the part. An inverted offsetting method accelerated with the depth buffer mechanism of GPU is introduced. An experimental system is implemented and some computational experiments are performed. Our system can detect the sphere contacting shape on the part surface with approximately 1 million polygons in a few seconds.