In order to perform format conversion between the formats of a plurality of files without any work by a user, a file system (100) stores a relation between a conversion originating file (130) and a conversion destination file (131), and synchronously with an issue of a file operation API, the format conversion processes are executed. A user performs only the tasks essential for an application, without taking into consideration various necessary format conversions (either one-step or multi-step). During the user task, it is not necessary to designate a conversion originating file and a timing of format conversion. A user can use always a latest conversion destination file.
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