1. A reduced Z-buffer generating method comprising: providing a reduced Z-buffer which contains representative Z values for determining a positional relationship between plural objects or fragments constituting the objects in a depth direction, the representative Z values indicating a depth from the point of view in units of predetermined pixel blocks each including a plurality of neighboring pixels; and comparing the Z values of respective rasterized pixels in a pixel block of an object or a fragment to be rendered with the representative Z value of a corresponding pixel block stored in the reduced Z-buffer, and updating the representative Z value of the pixel block with a farthest Z value if the farthest Z value indicates lying closer to the point of view than the representative Z value of the pixel block does, the farthest Z value indicating lying the farthest from the point of view among the per-pixel Z values in the pixel block.