A method and apparatus are provided for transmitting and receiving a plurality of individual tributary signals in multiplex form via a common line. At the transmitting end, the tributary signals, each of which has a similar initial frequency, are converted into a compound signal having a frame structure with a common data rate. At the receiving end, each individual tributary signal is retrieved from the compound signal with its initial frequency. A phase information signal portion including a respective phase difference between each tributary signal and the compound signal is formed and inserted into the compound signal in the shape of respective coded bits. The initial frequency of each tributary signal is recovered from the phase information signal portion included in the respective coded bits belonging to the respective tributary signals.