Abstract: A communications system adaptive echo canceller integrally contains a test far-end talker signal generator, an echo signal generator for generating an echo signal with a simulated echo path, a residual echo detector for detecting a residual echo based on the output of a subtracter, and a switching unit for switching an input and output signal of the echo canceller into an echo canceller test mode. Further, it provides a bypassing unit for, in a test mode, separating a call/talk connected to the echo cancellers from the echo cancellers and forming a bypass path between trunk circuits and a transmission circuit. The bypassing unit results in being able to realize a test for the echo canceller while a talk is alive.