PURPOSE: To detect the fault of a channel circuit by supplying the white noise signal of a prescribed level to an unused channel circuit and making it be an echo component by attenuating it, and detecting the level of a transmission side output signal by making said echo component be a transmission side input signal.

CONSTITUTION: The white noise signal from a white noise generation circuit 12 is inputted as a reception side input signal Rin to the unused channel circuit through a selector 20 and in addition, the white noise signal attenuated by an attenuation circuit 13 is inputted to it as a transmission side input signal Sin through the selector 19. In the channel circuit 11-1, as far as an echo canceller is normal, when the signal level of a channel CH1 is detected by a level detection circuit 14, it comes within a some low level based on a residual echo component. When the fault occurs, it goes to a higher level exceeding the some low level. Thus, the occurrence of the fault can be decided.