PURPOSE: To easily observably display a cut section by displaying the surface of a clipping face with a specific color when a Z-buffer coordinate value coincides with the clipping face in three-dimensional display based upon Z-buffer system.

CONSTITUTION: When the coordinate values of respective points in a three- dimensional substance are arranged on the front, back and inside of the clipping face is decided by an inter-clipping face deciding mechanism 1, the coordinate values are converted into a Z-buffer coordinate system by a Z-buffer coordinate converting mechanism 2. When the coordinates coincide with the clipping face, the point is adopted as a cutting section through a clipping face coincidence deciding mechanism 3 and the specific color stored in an internal register is adopted by a cut section forming mechanism 4 to form cut section data. Then, the cut section can be easily observably displayed with the specific color through a depth deciding mechanism 5, a Z-buffer writing mechanism 6, a frame buffer writing mechanism 7, and so on.