PURPOSE: To secure privacy and to simplify hardware by changing a synchronous pattern in each set, and relating the synchronous pattern to a scrambler pattern at the ratio of 1 to 1.

CONSTITUTION: A frame synchronizing signal pattern is sent by turning a switch 3. At the sending of the frame synchronizing signal pattern, the switch 3 is connected to the earth side, and at the sending of data, the switch 3 is connected to the input terminal 1 side, so that only the transmission data are scrambled and the prescribed frame synchronizing signal pattern is sent. The receiving side detects the prescirbed frame synchronizing signal pattern from the transmitted data and the data synchronized and scrambled on the basis of the detected pattern are restored to the original data through a descrambler. Therefore, a dif- ferent pattern is formed in each setting to secure privacy. On the viewpoint of hardware, only the scramble pattern may be controlled without using a frame pattern generating circuit independently to simplify the control.