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.