For transmitting transmission data generated by a transmission data generator of a communication terminal device, a packet unit determining unit [17] determines a packet unit for a lowest data communication rate for the transmission of the transmission data, among packet units (packet sizes) that can be recognized by a destination communication terminal device and that can be transmitted from the communication terminal device. A packet generator [18] packetizes the transmission data according to the packet unit determined by the packet unit determining unit [17], and transmits the packetized transmission data to the destination communication terminal device.