The distance between a moving character and a fixed character which are displayed in a pseudo-three-dimensional space on a display monitor screen is calculated by identifying one of polygons which make up the fixed character and which contains a point of planar coordinates representing the position of the moving character, and calculating the distance between the moving character and the polygon which has been identified. A polygon representing a mark associated with the moving character is applied to a plane of a polygon of the fixed character by determining a normal vector which is normal to the plane of the polygon at a predetermined position thereon, determining a first vector contained in the plane of the polygon perpendicularly to the normal vector, determining a second vector contained in the plane of the polygon perpendicularly to both the normal vector and the first vector, and determining the rotation matrix from the normal matrix, the first matrix, and the second matrix.