a copy management step of copying data of a server-side DB into a client-side built-in DB and the server managing contents of copying and a copying time;

a data synchronization step of synchronizing changed data between the mobile clients and the DB server with respect to a data change produced during a disconnection state of the mobile clients in accordance with a request from the mobile clients after a reconnection of the mobile clients; and

a conflict detection and resolution step of detecting an error that damages consistency of the data in the data synchronization process and resolving the detected error;

wherein in the copy management step,

the mobile client

(i) checks out from the server (a) main object disjoint fragments for a specified main object and (b) at least a subsidiary object different from and associated with the main object, and

(ii) copies the main object disjoint fragments into its own DB for use during the disconnection state, and

the server

(iii) allows a master copy of the checked-out subsidiary object maintained at said server to be available for downloading and updating by mobile clients other than the mobile client checking out the checked-out subsidiary object, wherein copies of the checked-out subsidiary object exist not only at the server and said mobile client, but also at other mobile clients, and

(iv) makes the checked-out main object fragments maintained at said server unavailable for loading or updating by the other mobile clients, wherein copies of the checked-out main object fragments exist only at the server and said mobile client, and not at any other mobile clients.