PURPOSE: To use the wiring of the parallel data also to the transfer of the serial data in a system which carries out the transfer of both parallel and serial data.

CONSTITUTION: In a system where a wiring for parallel data is provided between a parallel data transmitting part 1 and a parallel data processing part 2, a serial data transmitting part 3 and the part 1 are connected to the parallel data wiring via a switch means 4. At the same time, a serial data processing part 5 which receives the serial data is connected to the parallel data wiring in parallel to the part 2. Meanwhile a transmission control part 6 is added to selectively actuate the parts 1 and 3 and produces a control signal to control the switching operation of the means 4 and a transmission mode signal to distinguish the parallel data from the serial data. Furthermore a reception control part 7 selectively actuates the parts 2 and 5 in accordance with the transmission mode signal.