For synchronization signals of zeroth and first series, a synchronization signal processing system comprises synchronization trunk device of the zeroth and the first series (77(0), 77(1)) for producing particular signals of the zeroth and the first series, respectively, and being exclusively operable in master and slave states. When used in a switching center of a low hierarchy, each trunk device of a local series, which is one of the zeroth and the first series, is supplied as three input signals with the synchronization signals of the local series and a different series with highest (1) and middle (2) degrees of priority and the particular signal of the different series with a lowest (3) degree of priority, selects as a normal signal according to the degrees one of the synchronization signals in the master state and one of the three input signals in the slave state that are normal, and generates as the particular signal of the local series a regenerated synchronization signal with its phase adjusted to the normal signal. A relay trunk device of the local series (95(0), 95(1)) is supplied with the particular signals of the local and the different series with higher (1) and lower (2) degrees of priority and selects as a phase synchronized synchronization signal of the local series according to the higher and the lower degrees one of the particular signals that is normal. In a switching center of a highest hierarchy, the three input signals may be a synchronization signal of the local series produced in a lower hierarchy switching center, the phase synchronized synchronization signal of the different series, and the particular signal of the different series.