Abstract: In a mapping circuit of a CRT display device in accordance with the present invention, vertex calculating means (2 and 3) evaluates corresponding relations between vertexes of a three-dimensional polygon and vertexes of a two-dimensional polygon and inclination calculating means (4 and 5) calculates inclination of the line segments connecting the respective vertexes of the three-dimensional polygon and inclinations of the line segments connecting the vertexes of the two-dimensional polygon. Vertexes interpolating means (6 and 7) performs interpolation between the respective vertexes of the three-dimensional polygon and also performs interpolation between the respective vertexes of the two-dimensional polygon. Then, using the interpolated points as start points or end points, start and end points interpolating means (9, 10, 11 and 12) performs interpolation between the respective start points and end points.