PURPOSE: To decrease the extension time of a reception control signal in a protection circuit to prevent production of an error signal in the reception control signal.

CONSTITUTION: A reception signal 101 from a transmission line is inputted to a separate circuit 1, in which the signal is separated into a data main signal 103 and a control signal 102. Moreover, the reception signal 101 is inputted to a detection circuit 2, in which a bit error is detected. The detected bit error is converted into a bit error rate by a counter circuit 3. The bit error rate is inputted to a protection circuit 4, in which the bit error rate is used to protect the control signal from the separation circuit 1 in response to the bit error rate. That is, when the bit error rate of the control signal 102 is low, number of times of repetition of protection is reduced.