A method of generating a lower layer data block in a protocol layer of a transmitting side in a wireless communication system, the method comprising: generating a protocol data unit 2, PDU 2, which includes a service data unit 2, SDU 2, and a header 2; and generating a PDU 3 which includes a SDU 3 and a header 3, characterized by: generating the PDU 2 such that the header 2 includes a size change indicator 2 indicating whether a size of the SDU 2 is identical with a size of SDU 1 included in a PDU 1; and generating the PDU 3 such that the header 3 includes a size change indicator 3 indicating whether a size of the SDU 3 is identical with the size of the SDU 2, wherein the header 2 further includes a length field 2 indicating the size of the SDU 2 if the size change indicator 2 indicates that the size of the SDU 2 is different from the size of the SDU 1 included in the PDU 1; and the header 2 does not include the length field 2 if the size change indicator 2 indicates that the size of the SDU 2 is identical with the size of the SDU 1 included in the PDU 1, wherein the header 3 further includes a length field 3 indicating the size of the SDU 3 if the size change indicator 3 indicates that the size of the SDU 3 is different from the size of the SDU 2; and the header 3 does not include the length field 3 if the size change indicator 3 indicates that the size of the SDU 3 is identical with the size of the SDU 2. generating a protocol data unit 2, PDU 2, which includes a service data unit 2, SDU 2, and a header 2; and generating a PDU 3 which includes a SDU 3 and a header 3, characterized by: generating the PDU 2 such that the header 2 includes a size change indicator 2 indicating whether a size of the SDU 2 is identical with a size of SDU 1 included in a PDU 1; and generating the PDU 3 such that the header 3 includes a size change indicator 3 indicating whether a size of the SDU 3 is identical with the size of the SDU 2, wherein the header 2 further includes a length field 2 indicating the size of the SDU 2 if the size change indicator 2 indicates that the size of the SDU 2 is different from the size of the SDU 1 included in the PDU 1; and the header 2 does not include the length field 2 if the size change indicator 2 indicates that the size of the SDU 2 is identical with the size of the SDU 1 included in the PDU 1, wherein the header 3 further includes a length field 3 indicating the size of the SDU 3 if the size change indicator 3 indicates that the size of the SDU 3 is different from the size of the SDU 2; and the header 3 does not include the length field 3 if the size change indicator 3 indicates that the size of the SDU 3 is identical with the size of the SDU 2. generating the PDU 2 such that the header 2 includes a size change indicator 2 indicating whether a size of the SDU 2 is identical with a size of SDU 1 included in a PDU 1; and generating the PDU 3 such that the header 3 includes a size change indicator 3 indicating whether a size of the SDU 3 is identical with the size of the SDU 2, wherein the header 2 further includes a length field 2 indicating the size of the SDU 2 if the size change indicator 2 indicates that the size of the SDU 2 is different from the size of the SDU 1 included in the PDU 1; and the header 2 does not include the length field 2 if the size change indicator 2 indicates that the size of the SDU 2 is identical with the size of the SDU 1 included in the PDU 1, wherein the header 3 further includes a length field 3 indicating the size of the SDU 3 if the size change indicator 3 indicates that the size of the SDU 3 is different from the size of the SDU 2; and the header 3 does not include the length field 3 if the size change indicator 3 indicates that the size of the SDU 3 is identical with the size of the SDU 2.