PURPOSE: To recognize multi-frame synchronization without a delay in protection of multi-frame synchronization even for an optional number of backward protection stages with respect to the multi-frame synchronization protection circuit.

CONSTITUTION: The multi-frame synchronization protection circuit having a comparator circuit 2, a shift circuit 4 having a shift stage number required for forward and backward protection, and an alarm generating circuit 6 providing an output of an alarm signal is response to each forward protection output signal is characterized to be provided with a 1st circuit 8 outputting an output signal of the comparator circuit 2 to an input of a 1st shift stage, a 2nd circuit 10 outputting an output signal of the comparator circuit 2 only when the alarm signal is generated from the alarm generating circuit 6, and a 3rd circuit 12 applying the output signal of the forward protection of the 1st shift stage and an output signal of the 2nd circuit 10 to an input of a 2nd shift stage of the shift circuit 4.