A user terminal apparatus which constitutes a part of a thin client system which transfers data to a file server and stores the data therein, comprising: a secondary storage device; a cache memory; a file access control unit adapted to judge a class of an I/O request to the secondary storage device from an application; and first and second redirection processing units (212, 213) adapted to redirect the I/O request, characterized in that, if the file access control unit judges that the I/O request from the application is a double-write-enabled process and is a write request, the file access control unit transfers the I/O request to the first and second redirection processing units (212, 213), wherein the application of the double-write-enabled process realizes at least one of system revision data, an OS update or an application software update, and the first and second redirection processing units (212, 213) write data corresponding to the I/O request into different portions in the cache memory, respectively; and if the file access control unit judges that the I/O request from the application is not the double-write-enabled process but a write-enabled process, and is the write request, the file access control unit transfers the I/O request to the second redirection processing unit, and only the second redirection processing unit writes data corresponding to the I/O request which is the write-enabled process and is the write request, into the cache memory; wherein when the user terminal apparatus is shut down, the second redirection processing unit writes the data written to the cache memory by the second redirection processing unit, into the secondary storage device; and if the file access control unit judges that the I/O request from the application is a write-disabled process and is the write request, the file access control unit transfers the I/O request to the first redirection processing unit, and only the first redirection processing unit writes data corresponding to the I/O request which is the write-disabled process and is the write request, into the cache memory. a secondary storage device; a cache memory; a file access control unit adapted to judge a class of an I/O request to the secondary storage device from an application; and first and second redirection processing units (212, 213) adapted to redirect the I/O request, characterized in that, if the file access control unit judges that the I/O request from the application is a double-write-enabled process and is a write request, the file access control unit transfers the I/O request to the first and second redirection processing units (212, 213), wherein the application of the double-write-enabled process realizes at least one of system revision data, an OS update or an application software update, and the first and second redirection processing units (212, 213) write data corresponding to the I/O request into different portions in the cache memory, respectively; and if the file access control unit judges that the I/O request from the application is not the double-write-enabled process but a write-enabled process, and is the write request, the file access control unit transfers the I/O request to the second redirection processing unit, and only the second redirection processing unit writes data corresponding to the I/O request which is the write-enabled process and is the write request, into the cache memory; wherein when the user terminal apparatus is shut down, the second redirection processing unit writes the data written to the cache memory by the second redirection processing unit, into the secondary storage device; and if the file access control unit judges that the I/O request from the application is a write-disabled process and is the write request, the file access control unit transfers the I/O request to the first redirection processing unit, and only the first redirection processing unit writes data corresponding to the I/O request which is the write-disabled process and is the write request, into the cache memory.