A volume group is a group of two or more copy-source volumes and two or more copy-destination volumes. A computer system stores access rate information, which is information denoting an access rate for a copy-source volume at respective time periods. The computer system estimates the access rate for a future target period for each copy-source volume based on information denoting the access rate of each copy-source volume for one or more past target periods in a predetermined period from among the access rate information. The computer system determines the start timing of a copy for each copy-source volume based on the result of this estimation. A start time and an end time of the past target period are the same as the start time and the end time of a future target period.