PURPOSE: To realize that a necessary secondary power source is supplied and that the functions of a X21 interface are added to a terminal device which does not have the functions within a X21 interface part and by a small circuit scale without changing the whole of the device till when what is called X21 interface part which is the interface of a data terminal device DTE and a line terminal device DCE transmits a control impossible not ready signal of a prescribed bit length to an opposite party device DCE from when the power source of a self device DTL is turned off.

CONSTITUTION: Within a X21 interface part, an interface circuit 1, an interface control part 2 at the time of turning off a power source, a power source turning off detection circuit 3 and a preliminary power source supply part 4 are provided. When a self-device DTE becomes the state of power source turning off from a normal state, the interface circuit 1 is made to operate by power source current generated automatically and supplied by the prelimary power source supply part 4 and the face that the self-device DTE is in a control impossible not ready state is notified to an opposite device DCE by the signal of a prescribed bit length.