PURPOSE: To judge a trouble of an echo canceler or the disconnection of a line in a communication unable state by detecting only a pseudo echo in a look-back mode to confirm the normal working of an echo canceler.

CONSTITUTION: In a loop-back mode a switch 3 is set at a ground level with a switch 7 connected to a bit inverting circuit 6 and an estimated coefficient is fixed by means of a switch 13. Under such conditions, a pseudo echo signal produced by an echo canceler is supplied to a subtraction circuit 4 and an inverted pseudo echo signal is supplied to a wave detecting circuit 5. The pseudo echo signal undergone the wave detection and binary coding through the circuit 5 is supplied to an interface part 8 after the bit inverted by the circuit 6. The part 8 compares the transmission signal with the received pseudo signal. Then the normal working of the echo canceler is decided when the coincidence is obtained between said both signals. Thus the loop-back mode is through and a normal working state is reset.