PURPOSE: To confirm the normal operation according to the protocol based on a D-channel communication protocol by utilizing a signal being the result of inserting a prescribed test data to an information frame of an XID frame for system parameter negotiation having a global terminal identification number as a transmission signal.

CONSTITUTION: A transmission/reception circuit 1 inserts an optional test data to an information field of the XID frame for system parameter negotiation having a global terminal identification number and transmits the result, the XID frame is received as it is by the transmission/reception circuit 1 via a selection circuit 5, and the test data in the reception information frame and the sent test data are compared and collated by the firmware 6. The value of command/response bit in the address field is received with loopback similarly at transmission and reception and it is not required to allocate a terminal identification number in advance to a D-channel protocol processor 8. Thus, it is not required to set the test mode for the purpose of loopback test to the titled processor but the logic to which the titled processor to be followed is tested during the normal communication.