PURPOSE: To simplify the circuit configuration.

CONSTITUTION: A frame data inputted to a terminal 13 is recovered into a CMP signal to be logic 'H' in a n-th multi-frame by a frame coincident point detection circuit 1 and the signal enters a ROM 6. On the other hand, a protective stage number is changed by a selector signal M in order to implement synchronization protection in response to various multiplex systems. A (n+/≠n-) signal and a (OK+/NG-) signal are sent from the ROM 6 to a ROM 3 and a frame synchronization point is detected from the inputted frame data.