A signal transmission method is provided, wherein, when a transmitter transmits a signal to a receiver, a transmitter processor multiplexes a plurality of data for a plurality of connections to be transmitted to one specific receiver, and constitutes a multiplexed medium access control (MAC) packet data unit (PDU) containing a MAC header part for receiving the multiplexed data, and a MAC PDU payload containing the multiplexed data, and the transmitter transmits the multiplexed MAC PDU to the specific receiver. The MAC header part includes a first header part containing a generic MAC header, and a second header part containing an extended header indicator serving as a 1-bit indicator for indicating the existence of an extended header (EH) for each flow ID for each of the plurality of connections. The MAC header part further includes a third header part which selectively contains the extended header for each flow ID in accordance with the extended header indicators for each flow ID.