In a synchronization method and apparatus in a data transmission using a transparent transmission line, a periodic amplitude modulation is applied, or a zero-point is inserted, as a synchronizing signal to a transmission signal point on a transmission side at a preceding stage of a transparent transmission line. Also, in case of a frame synchronization, a power value of a received signal is calculated on a reception side of the above-mentioned transparent transmission line and the synchronizing signal is vectorized using the power values whose phases are different from each other by 90 degrees on a time axis.