A communication system capable of a stable communication operation between a base station device and a communication terminal device. A UE is configured to communicate with an MeNB directly or through a SeNB. The UE is set to transmit, to the MeNB and the SeNB, transmission data addressed to the MeNB when an amount of the transmission data exceeds a predetermined threshold. The UE is set to transmit the transmission data not to the SeNB but to the MeNB when the amount is smaller than or equal to the threshold. The threshold is changed so that the transmission data is transmitted to the MeNB and the SeNB, when the SeNB is set to communicate with the UE with radio resources periodically allocated and the amount of the transmission data is smaller than or equal to the threshold.