Methods are in use for saving data stored in a volatile memory to a non-volatile memory during a power outage in a storage system so that the data on the volatile memory is not lost. In the related art, the entire data saved to the non-volatile memory is read and written in the volatile memory when electric power supply is restored. Then, an operation as the storage system is resumed and an access request from a host is accepted. In the related art, the data saved in the non-volatile memory has to be read in entirety, and thus time is required until resumption of the access request acceptance and availability is impeded. According to the invention, a cache directory and cache data are saved to a non-volatile memory during a power outage and the cache directory is copied from the non-volatile memory to a volatile memory, a storage location of the data whose storage location in the cache directory prior to the saving is the volatile memory is changed to the non-volatile memory, and host I/O processing is initiated when electric power is restored in order to address this problem.