Abstract: A covering object (42) that covers a three-dimensional object (40) is provided within an object space. Every time a game-clear condition is satisfied, the display/non-display state of a polygon forming the covering object (42) is sequentially switched. If a moving body (44) moves to specify a region (36), the polygons of that region (36) are set to non-display. Alternatively, polygons that form the covering object (42) are sequentially set to non-display, on the basis of a criterion such as the ratio of correct answers in a quiz game, whether or not a mini-game has been cleared, or whether or not the player has won in Mah-Jongg. The transparent/nontransparent state of the covering object could equally well be switched sequentially, or the translucency thereof could be sequentially changed, or a texture mapped onto the three-dimensional object could be sequentially changed.