An internode put requesting unit detects a time-out with respect to a put request issued to the next node in the order of a multiplexing chain and notifies a put/get executing unit of the time-out. The put/get executing unit sends an error to the previous node in the order of the multiplexing chain or a client and instructs a put-failed-data synchronizing unit to synchronize data failed to be put, and the put-failed-data synchronizing unit performs a synchronization process. A primary makes other put requests wait until completion of the synchronization process. Furthermore, when having received the error, the client issues a get request to the tail end of the multiplexing chain.