PURPOSE: To contrive low power consumption of the device by lowering the number of revolutions of a motor when a power supply control signal is outputted without necessity of an operation of one part at the time except at the read time, write time and seek time in the hard disk device.

CONSTITUTION: The hard disk device 3 is equipped with an MPU 1 for recognizing unnecessity of the operation of one part at the time except the read time, write time and seek time in the device, a power management means 4 and an I/O control means 2 to be as an interface between the MPU 1 and the hard disk device 3. In addition, a control part 31, a signal processing part 32, a servo part 33 and a power supply control part 34 are equipped, and based on the power supply control signal, the number of revolutions is lowered.