PURPOSE: To attain the efficiency of real time of the system by providing a means for changing the multiplexity of data transfer in a data multiplex transfer device for the system using a bus system transferring data in time division to improve the transfer efficiency between devices.

CONSTITUTION: In case of the duplex transfer, a gate of a data selector circuit 3 is operated by a timing signal formed by a data transmission timing forming circuit 1 to transmit data, while a strobe forming circuit 2 forms and transmits a strobe signal synchronized with the data and the duplex transfer is informed to the reception side by a CNT read additionally. Then the reception side recognizes the duplex transfer by the CNT read, a strobe extraction circuit 6 extracts the strobe signal ST, distributes respectively the 1st strobe signal and the 2nd strobe signal to the 1st data register 4 and the 2nd register 5 and then the duplex transfer of data is attained.