When a plurality of coplanar shapes are rendered over one another by performing a hidden surface removal operation using the Z value, a reference plane is specified, and the Z value of each point of a shape to be rendered on the reference plane is uniquely calculated based on the Z value of the rendering start point, the Z value gradient dZdx for the X direction, and the Z value gradient dZdy for the Y direction. Thus, any coplanar shapes will have the same Z value for each point shared therebetween. Therefore, if the rendering process is performed under such a rule that a shape is overwritten when the Z value of the new shape is less than or equal to the current Z value, it is ensured that the previously-rendered shape is always overwritten with a later-rendered, coplanar shape. Thus, it is possible to prevent a phenomenon that some pixels of a later-rendered shape that are supposed to be displayed are not displayed.