PURPOSE: To monitor A/D and D/A conversion as to all channels by exercising the monitor so that the level difference between a transmitted ultiplex pulse- amplitude-modulated signal and transmitted multiplex pulse-code-modulated signal is held below a prescribed value.

CONSTITUTION: The input analog signal of a channel to be monitored is pulse- amplitude-modulated PAM and then multiplexed to obtain transmitted multiplex PAM signal 501 for the monitor. The analog output signal is pulde-code-modulated PCM and multiplexed to obtain transmitted multiplex PCM signal 502. Then, the monitor device obtains modulated analog signals Va and Vb of signals 501 and 502. So that level difference Va-Vb between the above-mentioned signals VQ and Vb will be less than a prescribed value, deciding circuit 516 exercises the superivision, performing efficient A/D and D/A conversion with regard to all channels of the PCM multiplexer.