A communication terminal device connected to a network and performing data communications between another communication terminal device through the network, includes 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 a self device to the slave unit operation mode based on the proper information, while the self device is operating by the master unit operation mode. The master unit operation mode is the operation mode of the communication terminal device operating as a master unit, and the slave unit operation mode is the operation mode of the communication terminal device operating as a slave unit.