1. A rendering processing unit for rendering three-dimensional images on a two-dimensional screen, the three-dimensional images being each made up of a plurality of primitives, comprising: a primitive buffer in which a plurality of attribute data sets are written in association with relevant primitives, each attribute data set representing attributes of one of the plurality of primitives; and a tester that compares the plurality of attribute data sets in said primitive buffer with each other to categorize the plurality of primitives into a first group of primitives that are to be displayed on the two-dimensional screen and a second group of primitives that are not displayed thereon; the rendering processing unit being configured to render the first group of primitives and not to render the second group of primitives that are categorized out by said tester.