A game system in which game software produced to be used in stationary game machines can be used in portable game machines. The game system includes a stationary game machine, a portable game machine, and a memory card connectable to both the stationary and portable game machines. Image data contained in game software produced for the stationary game machine shows an image in the form of free-form surface. The stationary game machine converts the image data to show the image in the form of a polygon according to processing performance of the portable game machine, and stores the converted image data and a game program executable by the portable game machine onto the memory card. The portable game machine executes the game program stored on the memory card, and generates and displays a display image from the image data stored on the memory card.