PURPOSE: To shorten time for completing multiframe synchronization of input data that particular bits of each frame become a synchronous signal for every N (N; integer) frames (one multiframe).

CONSTITUTION: An extract circuit 3 for extracting specified bits for 2×N frames and a register 4 for storing the particular bits are installed in the frame synchronization detecting circuit. Further, a comparator 5 that compares bits themselves separated by the distance equivalent to N frames regarding particular bits stored in the register 4 and a counter 6 for detecting the synchronous signal based on the result of comparison. In this case, when a counted value by the counter 6 reaches a predetermined value, the bit and synchronous signal can be specified and it is considered that multiframe synchronization has been completed.