A game system utilizes a first game machine and a second game machine. The backup data obtained through playing a first game on the first game machine is memorized in a first game cartridge. The first game cartridge is attached to an extension adapter, and the extension adapter is in turn mounted on a controller for the second game machine. Accordingly, the backup data of the first game machine is taken into the second game machine. The second game machine is attached with a second game cartridge. The second machine executes a second game by processing the backup data according to a second game program set up in the second game cartridge.