This invention includes: a step (S1) of receiving a snapshot creation request from a client computer; a step (S3) of obtaining, upon reception of the snapshot creation request, a usage ratio of a second volume by the update differences; a step (S2) of obtaining a use history of the update differences in the second volume of the user; a step (S4, S5) of choosing, based on the usage ratio and the use history, one from differential snapshot in which the update differences of the first volume are written in the second volume and volume snapshot in which a third volume is synchronized with the first volume; and a step (S6, S7) of executing the chosen one of differential snapshot and volume snapshot. As a result, it is possible to economically use a differential volume by using differential snapshot in combination with volume snapshot.