To provide a data input/output device to enable distribution and unifomalization of loads and to dispense with stand-by of use of a bus by a certain group of devices for a long period.

When the bus is requested by one 10a of interface cards, a bus request signal is temporarily withdrawn by the other 10b. Thus, when data transfer is completed by one 10a of the interface cards, a right to use the next bus is provided to SCSI cards 8a, 8d to a mediation circuit of the bus. The bus request signal is outputted again in response to completion of the transfer by one interface card 10a and the right to use the bus is waited to be secured by the interface card 10b by which the bus request signal is withdrawn.