The present invention relates to a mobile communication system in which data is transmitted and received through a wireless transmission line between a mobile station device and a network-side device. A receiving-side device includes a retransmission request transmitter configured to transmit, to a transmitting-side device, a retransmission request for requesting retransmission processing on each data unit of the data, when a reception error is detected in the data unit. The transmitting-side device includes a retransmission time storage configured to store a retransmission time at which the retransmission processing is performed on the data unit; and a retransmission controller configured to control so as not to perform the retransmission processing on the data unit related to a received retransmission request, when the retransmission time of the data unit is stored and a difference between the retransmission time and a current time is shorter than a retransmission prohibition period.