A storage system including a storage, has a first power supplier for supplying electronic power, a second power supplier for supplying electronic power when the first power supplier not supplying electronic power to the storage system, a cache memory for storing data sent out from a host, a non-volatile memory for storing data stored in the cache memory, and a controller for writing the data stored in the cache memory into the non-volatile memory when the second supplier supplying electronic power to the storage system, for stopping the writing and for deleting data stored in the non-volatile memory so until a free space volume of the non-volatile memory being not less than a volume of the data stored in the cache memory when the first supplier restoring electronic power to the storage system.