This paper deals with a new calculation algorithm of Z-map representation with graphics hardware to perform geometric simulation of NC machining process. Graphics hardware (GPU) is a device designed to render 3D computer graphics. In previous studies, it is proposed that shape of machined workpiece is estimated by referring depth information on rendered image by GPU. However, such depth information usually contains calculation error that cannot be corrected, because GPU cannot deal with curved surface directly and requires approximation to a polyhedron. In this paper, a new geometric calculation method based on GPU functions is proposed in order to estimate accurate depth information from rendering process. In the proposed method, pixel shader in GPU estimates a distance between the curved surface and the approximated polyhedron in modeling coordinate system and corrects the depth information on frame buffers.