In a device and method for minimizing transmission delay of low-rate data such as voice data and for enhancing the transmission efficiency in multiplexed data communication using AAL type 2 protocol the SSCS of the high-rate data side of an AAL type 2 converter is responsible for addition and elimination of a dummy code field, error correction, and division and reassembly of data unit. A dummy code field and restorative code field are attached to a user data unit which is supplied from the higher layer in a source device, whereby a data unit is prepared. The data unit is then divided into segments 1 to m of a fixed-length, 45 octets. Successive code fields 1 to m are attached to the segments 1 to m, whereby data blocks 1 to m are prepared. The data blocks 1 to m are successively transferred to the AAL-2 converter. In a destination device, from the data blocks 1 to m, the user data unit is reassembled.