A cloud storage system includes a plurality of cloud storage modules for storing and managing data and a data encryption processing device. The data encryption processing device includes a priority manager for managing priorities on encrypting data of a plurality of cloud storage modules by using information on whether encryption processing for each of the plurality of the cloud storage modules is supported; and an encryption requester for selecting at least one cloud storage module on the basis of the priorities managed by the priority manager when receiving request of a data encryption, and performing, by the selected at least one cloud storage module, encryption after delivering the data to the selected cloud storage module.