To reduce the difference between the devices which transfer the data using bus in regard to the magnetic disk devices as well as a disk controller which sets the bus using right through the arbitration control.

A disk controller 11 and the magnetic disk devices 21, 22 and 23 perform the arbitration control to secure the using right of a bus 1, respectively. When the number of data transfer requests of a device connected to the bus 1 reaches its upper limit value, the device asserts a preference signal 2. If the signals 2 has its true state, the device connected to the bus 1 cannot join in the arbitration excluding a case where the device itself is asserting the signal 2.