Abstract: In a monitoring apparatus for monitoring digital equipment including an on-line multiplexer for multiplexing a plurality of input signals into a single output signal, an on-line demultiplexer for demultiplexing a single input signal into a plurality of output signals, and off-line multiplexer and demultiplexer substitutable for the on-line multiplexer and demultiplexer, respectively, a pattern signal generator supplies the off-line multiplexer and demultiplexer with a monitoring signal of a predetermined pattern identical with a synchronization signal included in each of the plurality of input signals and the plurality of output signals. The monitoring signal is circulated through the off-line multiplexer and demultiplexer and is supplied to a monitoring circuit as a circulated signal. The monitoring circuit detects presence or absence of the predetermined pattern in the circulated signal to monitor occurrence of failure in the off-line multiplexer and demultiplexer.