In a storage system in which a plurality of pieces of control software constituting a redundancy group are distributedly arranged in a plurality of storage nodes, control software in an active state out of the plurality of pieces of control software constituting the redundancy group receives a write request from a higher-level device. The control software in the active state writes data related to the write request by mirroring into a cache memory of a storage node in which the control software in the active state is arranged and a cache memory of a storage node in which control software in an inactive state belonging to the same redundancy group is arranged. The control software in the active state sends a write completion response to the higher-level device, and redundantly stores the data written in the cache memories in a storage device.