Abstract: In a multiplexer and a usage thereof, a higher-order cross-connect of the multiplexer inputs a higher-order synchronous network signal from any of higher-order input channels to be cross-connected to any of higher-order output channels, a lower-order cross-connect inputs a lower-order signal, which can be multiplexed into the higher-order synchronous network signal, from any of lower-order input channels to be cross-connected to any of lower-order output channels, and a part of the higher-order output channels and a part of the lower-order input channels are mutually connected as well as a part of the lower-order output channels and a part of the higher-order input channels are mutually connected.