A communication terminal device (2 to 7) connected to a network and performing data communications with another communication terminal device (2 to 7) through the network, comprising: a storage configured to store proper information used as a criterion for switching an operation mode between a master unit operation mode and a slave unit operation mode; and circuitry configured to switch the master unit operation mode of the communication terminal device (2 to 7) itself to the slave unit operation mode based on the proper information, while the communication terminal device (2 to 7) itself is operating in the master unit operation mode, wherein the master unit operation mode is the operation mode of the communication terminal device (2 to 7) when the communication terminal device is operating as a master unit, and the slave unit operation mode is the operation mode of the communication terminal device (2 to 7) when the communication terminal device is operating as a slave unit, wherein the proper information includes judging criterion information used as the criterion which judges the priority of the master unit operation mode of the communication terminal device (2 to 7) itself, characterized in that the circuitry is further configured to: monitor whether or not the other communication terminal device (2 to 7) is another coexisting master unit which operates in the master unit operation mode, when the communication terminal device (2 to 7) itself operates in the master unit operation mode; acquire the judging criterion information of the other communication terminal device (2 to 7) detected as the other master unit operating in the master unit operation mode; and switch, while the communication terminal device (2 to 7) itself is operating in the master unit operation mode and when the other communication terminal device (2 to 7) is detected as another coexisting master unit which operates in the master unit operation mode, the master unit operation mode of the communication terminal device (2 to 7) itself to the slave unit operation mode based on the judging criterion information of the communication terminal device (2 to 7) itself and the judging criterion information of the other communication terminal device (2 to 7). a storage configured to store proper information used as a criterion for switching an operation mode between a master unit operation mode and a slave unit operation mode; and circuitry configured to switch the master unit operation mode of the communication terminal device (2 to 7) itself to the slave unit operation mode based on the proper information, while the communication terminal device (2 to 7) itself is operating in the master unit operation mode, wherein the master unit operation mode is the operation mode of the communication terminal device (2 to 7) when the communication terminal device is operating as a master unit, and the slave unit operation mode is the operation mode of the communication terminal device (2 to 7) when the communication terminal device is operating as a slave unit, wherein the proper information includes judging criterion information used as the criterion which judges the priority of the master unit operation mode of the communication terminal device (2 to 7) itself, characterized in that the circuitry is further configured to: monitor whether or not the other communication terminal device (2 to 7) is another coexisting master unit which operates in the master unit operation mode, when the communication terminal device (2 to 7) itself operates in the master unit operation mode; acquire the judging criterion information of the other communication terminal device (2 to 7) detected as the other master unit operating in the master unit operation mode; and switch, while the communication terminal device (2 to 7) itself is operating in the master unit operation mode and when the other communication terminal device (2 to 7) is detected as another coexisting master unit which operates in the master unit operation mode, the master unit operation mode of the communication terminal device (2 to 7) itself to the slave unit operation mode based on the judging criterion information of the communication terminal device (2 to 7) itself and the judging criterion information of the other communication terminal device (2 to 7). monitor whether or not the other communication terminal device (2 to 7) is another coexisting master unit which operates in the master unit operation mode, when the communication terminal device (2 to 7) itself operates in the master unit operation mode; acquire the judging criterion information of the other communication terminal device (2 to 7) detected as the other master unit operating in the master unit operation mode; and switch, while the communication terminal device (2 to 7) itself is operating in the master unit operation mode and when the other communication terminal device (2 to 7) is detected as another coexisting master unit which operates in the master unit operation mode, the master unit operation mode of the communication terminal device (2 to 7) itself to the slave unit operation mode based on the judging criterion information of the communication terminal device (2 to 7) itself and the judging criterion information of the other communication terminal device (2 to 7).