In a game system such as a television game system, a television game machine main unit includes a protocol controller and each game operation device connected to the television game machine unit via a serial port also has a protocol controller. The television game machine main unit when connected to a game operation device which has a unique identifier, combines a communication protocol that transmits and receives data for each character with a communication protocol which transmits or receives data for two or more characters in a row and switches the communication protocol which corresponds to that game operation device based on the identifier thereof to minimize the amount of information required between the television game machine main unit and the game operation device.