PURPOSE: To discriminate quickly the frame synchronism establishment by inserting a CRC (cyclic redundancy check) code to each frame.

CONSTITUTION: A data of a terminal device is multiplexed at a multiplexing section 1, a frame signal is inserted to a frame signal inserting section 3, a CRC code is inserted at a CRC code inserting section 21, it is transmitted via an MODEM, the frame signal is detected from a receiving signl at a frame signal detecting section 6', a control section 22 performs CRC check, and when the signal is not coincident, the frame signal detecting section is reset, and when coincident, the CRC code is eliminated at a CRC code eliminating section 24, the signal is separated multiplexedly at a distribution section 7 and outputted to a terminal device via an interface.