PURPOSE: To satisfy the standards of the output voltage by the NTT K.K. with excellent S/N by using a changeover section to apply an element having a nominal impedance of a high speed digital transmission line thereby terminating the line when the high speed digital transmission line is not connected.

CONSTITUTION: When the high speed digital transmission lines l, 2 are not connected, changeover sections 6, 7 use the element 3 having the nominal impedance of the lines l, 2 to terminate them. Since the element 3 having the nominal impedance of the lines l, 2 terminates the line by the changeover sections 6, 7 even at the output terminal open state, the standards are satisfied by bringing the output voltage of the communication equipment to 0.6V when it is connected to the lines l, 2 thereby improving the S/N.