The present invention is devised so that snapshot data preparation processing does not end abnormally as a result of the pool region becoming full with saved data from the primary logical volume during snapshot data preparation. When the CHA receives a snapshot data preparation request, the CHA checks whether or not update information is present in the logical volume that is the object of this data preparation. If such information is present (YES in S51), a check is made from the number of slots in the in the volume control information and the updating amount count value in the update information in order to ascertain whether or not the proportion of the updating amount relative to the number of slots is equal to or greater than the threshold value in the threshold value control information (S52). If this proportion is equal to or greater than the threshold value (YES in S52), the update information is initialized (S53), and physical snapshot control is started (S54). If such is not the case (NO in S52), the update information is initialized (S55), and logical snapshot control is started (S56).