For at least one of storage unit, processor and cache memory which are I/O process-participating devices related to I/O command process, when a load of one or more I/O process-participating devices or a part thereof is a low load equal to or less than a predetermined threshold value, a processing related to a state of one or more of the I/O process-participating devices or a part thereof is redirected to another one or more I/O process-participating devices or a part thereof, and the state of the one or more I/O process-participating devices or a part thereof is shifted to a power-saving state.