Even if the version of the OS and DB is changed, occurrence of inconsistency in the correspondence relationship between the original version meta information and the new version real data is prevented. When a node managing a plurality of files is to return the version of the OS and DB of a plurality of files from the new version to the original version, the node deletes meta information A11 of the file A regarding such deleted file A, updates meta information B11 according to the updated real data B1 regarding such file B in which the real data has been updated, creates meta information based on real data of the added file D regarding such added file D, and thereby maintains the consistency in the correspondence relationship between the meta information of the respective original versions and the real data of the respective new versions regarding the respective files A, B, and D.