In a data center, when a server is disconnected from the Internet because of illegal manipulation of a file, valid files (not manipulated) are copied to a standby server and the processing is resumed after a short period of disconnection. According to the invention, a control program P10 performs manipulation checks for all files listed in a manipulation checklist. If any of them is manipulated, it issues a request to disconnect from the external network and a request to assign a standby server to a control program P20 of a management server. Upon receipt of a disconnection completion signal and the address of a standby server b0, the control program P10 copies valid files (not manipulated) to the standby server b0, and for invalid files, requests the copies of their backup files. After finishing copying, the management server connects the standby server b0 to the external network to resume processing.