A multiplexing apparatus is disclosed in which plural digital data are written into a memory and, stored in it. The data is read out in sequence by a clock to get a subsequent group of digital data. The memory is for storing digital data to be transferred from a certain high-speed circuit to another high-speed circuit and a control device exists for performing a writing and reading-out control of the digital data for the memory. The digital data, to be relayed from a prescribed high-speed circuit of plural high-speed circuits to another high-speed circuit is initially stored. A writing of the digital data of another high-speed circuit card is allowed from the memory means by the control device when this desired high-speed circuit card is being used. Further, when another high-speed circuit card is being used, a writing of the digital data of the prescribed high-speed circuit card is allowed, from the memory device through the control device to perform a high-speed relay of the digital data of plural high-speed circuits. This results in that even if the number of high-speed circuits are increased, the number of ICs are not changed and a relay of the high-speed digital data can be performed.