The present invention relates to a broadband wireless access system, and more specifically, to a structure of an efficient signaling header. A medium access control protocol data unit (MAC PDU) including a medium access control header (MAC header) is proposed; the MAC header including a flow identifier field, an extended header field, and a length field. When the MAC header is a signaling header, the flow identifier field is set to a specific value indicating that the MAC header is the signaling header, and the extended header field and the length field include signaling content information. But, when the MAC header is not the signaling header, the flow identifier field is set to value other than the specific value, the extended header field indicates whether or not an extended header is included in the MAC PDU, and the length field indicates the length of the MAC PDU.