PURPOSE: To shorten the setting time for device conversion and system reconstruction by providing a data bus converting circuit for converting a data bus, and a selection phase detecting circuit between a SCSI bus and each device.

CONSTITUTION: An ID setting device 2 between a SCSI bus 1 and each device is provided with a data bus converting circuit 3 and a selection phase detecting circuit 4, respectively. A first switching means of the data bus converting circuit 3 is operated by a first signal from the selection phase detecting circuit 4, the conversion of the data bus is executed, and a second switching means is operated by a second signal from the selection phase detecting circuit 4, and the conversion of the data bus is not executed. In such a state, the selection phase detecting circuit 4 detects a selection phase, and a first and a second switching means execute the conversion of the data bus. In the data bus converting circuit 3, conversion logic is determined in advance in accordance with its connecting position, thereby a prescribed device is selected in the result of conversion of the data bus.