A pseudo-inverse multiplexing/de-multiplexing apparatus and method are disclosed. The pseudo-inverse multiplexing apparatus maps a client signal to an OPUk-Xpv signal. The OPUk-Xpv signal has a payload area that can be segmented into a plurality of tributary slots and an overhead area into which frame configuration information related to the tributary slots is inserted. The pseudo-inverse multiplexing apparatus decides the number of tributary slots to be used to map client signals, according to a bit rate or bit tolerance of the client signals, and maps the client signals using the determined number of tributary slots. Accordingly, it is possible to map or frame a variety of client signals.