To provide an FAT file system in which rolling back to a pre-update status is automatically performed when hit reset is generated.

An FAT updating means 101 copies an FAT 112 and a node information table 114 on a non-volatile memory 110 on a main storage device 120, and updates an FAT and the node information table on the main storage device 120. When updating a file stored in a file area 115, a non-volatile writing means 104 writes the updated contents in an empty cluster by leaving the contents of the cluster corresponding to the update part of the file as they are. When closing the file, the non-volatile writing means 104 writes back the contents of the FAT and the node information table copied on the main storage device 120 in an FAT area 111 and an RDE(route directory entry) on the non-volatile memory 110.

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