PURPOSE: To prevent a pseudo synchronization by adopting a constitution that a gate is closed when a non-signal state is discriminated so as to make violation from a decoding circuit ineffective.

CONSTITUTION: A decoding circuit 1 outputs an NRZ code and a frame synchronizing signal. A non-signal state discrimination circuit discriminates whether or not a non-signal state exists from the CMI code input and outputs the discrimination result. A synchronization protection circuit 2 detects a fame synchronizing signal gated by a gate circuit 4 and confirms violation, and outputs a synchronization establishing display signal. The CMI code input is supervised at every prescribed period and when a non-signal state discrimination circuit 3 discriminates the absence of non-signal, the gate circuit is opened to make a frame period signal of the decoding circuit 1 effective and when the presence of non-signal is discriminated, the gate circuit is closed to make the violation from the decoding circuit ineffective thereby preventing the pseudo synchronization.