PURPOSE: To surely detect synchronism even if a violation detection signal goes to the high level in an irregular position, by comparing a ripple carry signal of a counter which counts the bit length of a frame and the violation detection signal with each other to detect synchronism.

CONSTITUTION: If synchronism is not established when a violation detection signal 20 is inputted, an AND circuit 2 is opened and a load signal is outputted to a counter 4 and the number of bits in one frame is set. Meanwhile, if synchronism is established, an AND circuit 1 is opened at the time of turning-on of a ripple carry signal 7 of the counter 4 and the number of bits in one frame is loaded to the counter 4. A comparing circuit 5 outputs a synchronism establishing signal 22 if the carry signal 7 from the counter 4 is turned on when the detection signal 20 is inputted, but a step out signal 8 is turned on unless signals 7 and 20 are simultaneously inputted.