Abstract: A frame synchronizing signal insertion system at a transmitting end has a plurality of multiplexers. Frequency-divided frame synchronizing signals are produced by frame pattern generators of the plurality of multiplexers, respectively. These frequency-divided frame synchronizing signals which have different patterns are logic-ORed by a multiplex gate so as to obtain a composite frame pattern. When a composite multiplex signal having the composite frame pattern is transmitted to a receiving end and the composite frame pattern does not match a predetermined pattern, the receiving end can detect a malfunction of any one of the multiplexers at the transmitting end, and transmits an alarm signal or the like to indicate malfunctioning of the corresponding multiplexer.