A game system includes a game machine in which first and second cartridges can be simultaneously inserted. In a ROM of the first cartridge, a first game program is stored. When data is stored in a reserved area in a flash memory of the first cartridge, the first game program can be executed using the data. For example, when both of the cartridges are inserted simultaneously in the game machine, according to an instruction of a player, a writing program stored in the ROM of one of the cartridges is executed, and additional data stored in the ROM of the second cartridge, for example, image data used for generating a game screen is written into the flash memory of the first cartridge.