To provide a multiplex transmission system that realizes multiplex transmission capable of transmitting data at a high speed without the need for increasing a transmission clock frequency.

The multiplex transmission system where data are transmitted/ received between a master communication port 1 of a master unit and slave communication ports 2 of slave units through serial data lines 3, is provided with n-sets (n is 2's power) of serial data lines 3 and a clock line 4. Each of the master communication port 1 and the slave communication port 2 is provided with a clock generating circuit 10, a parallel/serial conversion circuit 9, an encode circuit 8 that transmits converted serial data to n-sets of the serial data lines 3 as 1-bit data synchronously with a clock signal, a decode circuit 6 that converts the data sent via the serial data liens 3 and the clock line 4 into n-bytes of serial data on the basis of the clock signal, and a serial/parallel conversion circuit 5.