PURPOSE: To elevate transfer capacity, by using plural buses for connecting logical devices, instead of one bus so that a data can be transferred even when other logical device is transferring data.

CONSTITUTION: When a logical device 10 and a logical device 11 are transferring data by use of a bus 1, in case when it is desired to transfer data between a logical device 12 and a logical device 13, the logical device 12 and the logical device 13 confirm whether a use signal showing "using" is sent out to a bus using signal line 5 or not. If it is sent out, subsequently, whether the use signal showing "using" is sent out to a bus using signal line 6 or not is confirmed, and if it is not sent out, a bus 2 corresponding to this bus using signal line 6 is obtained, and the data is transferred. If it is sent out, furthermore, whether a use signal showing "using" is sent out to a bus using signal line 7 or not is confirmed, by which the data is transferred by utilizing an idle bus.