a method for retransmitting data between two sides including a reception side and a transmission side in a mobile communication system including one or more mobile stations and one or more radio networks includes the steps of : by the reception side , storing data received from the transmission side in a first storage unit ; as a result of an error - checking procedure , if the data are erroneous , requesting the transmission side to retransmit the data ; by the transmission side , transmitting to the reception side a first information related to retransmission and retransmitting the requested data ; by a combination unit included in the reception side , combining the retransmitted data with the data stored in the first storage unit ; if the combined data are not erroneous , clearing the data and the retransmitted data from the first storage unit and transmitting the combined data to a first upper layer included in the reception side ; and in response to an ack signal from the reception side representing that normal data are received , clearing by the transmission side the retransmitted data from a second storage unit .