PURPOSE: To monitor a failure in a device at all times, by providing a monitor signal in a vacant channel in SSB.FDM/PCM.TDM conversion.

CONSTITUTION: An analog signal 30 having a specific frequency is inserted at the outside of band of an SSB-FDM signal as a monitor signal, under the condition that the channel number processed at the inside of a transmultiplexer is larger than the channel number of SSB-FDM and PCM-TDM signals based on the sampling signal in encoding the SSB-FDM signal from a BPF1. Further, a monitor signal produced in a vacant channel of a time division multiplex signal before being speed-converted 7 into the PCM-TDM signal as a digital signal of a nonlinear code is monitored. As a result, a failure in the transmultiplexer is monitored.