According to the present invention, in cases where a CHA function and a DKA function are mounted within a single package, a battery power supply that is used during the occurrence of power supply trouble is effectively utilized so that the supply of power can be separately controlled for each function. A CHA part and DKA part are disposed in a single control package. When trouble such as a power outage is detected, the CHA part blocks access requests from the host, and initiates end processing. When the end processing of the CHA part is completed, the package internal power supply control part stops the clock supply to the CHA part. Then, when the DKA part completes destage processing, the package internal power supply control part stops the supply of power to the DKA part. The power consumption of the package is lowered in stages in accordance with the progress of the end processing.