It is determined whether the importance of an object storage node is equal to or larger than a predetermined importance and the reliability of the object storage node is equal to or larger than a predetermined reliability, the object storage node being a storage node set as an object among N storage nodes that are members of a storage cluster, N being an integer equal to or larger than 3. When the determination result is true, reintegration of the object storage node is performed. The importance of the object storage node depends on highness of availability when assuming that the object storage node has left the storage cluster. The reliability of the object storage node depends on the tendency of operation of the object storage node.