When a public line is used for a remote copy function of storage in a disaster recovery (DR) system, the online performance is degraded due to the increase and decrease of the line utilization rate. Therefore, it is desired to change the remote copy function depending on the line utilization rate. Also, there is the possibility of data loss if disaster occurs during the copy between storage devices. Therefore, it is desired to perform the processing to detect the data status immediately before the occurrence of the disaster. By switching the synchronous transmission and the asynchronous transmission in accordance with the increase and decrease of the response time of the remote copy function between storage devices, the online performance of the primary site can be maintained without being influenced by the line utilization rate, and the data loss can be detected based on the used remote copy function.