Abstract: Each one of intermediate control devices and n-number of disk drives are daisy-chain connected by control-information signal lines in each of downward and upward directions. Serial communication means (a driver and a receiver) sends and receives control information in the form of a bit serial between the intermediate control devices and each drive via the control-information signal lines in the downward and upward directions in accordance with a serial interface. This makes it possible to diminish the size of a magnetic disk apparatus by reducing the number of control-information signal lines and the numbers of drivers and receivers, and to cut down on power consumption so as to lower cost.