A file storage apparatus capable of restoring integrity of file management information even when a power supply abnormality occurs without lowering the write speed. When updating meta data stored in an HDD, log data for reconstructing the meta data after update from the meta data before update is written into a non-volatile RAM (NVRAM), then, after this writing is completed, the update is executed. Accordingly, even when the update use meta data temporarily stored in a cache memory is partially lost due to trouble such as a power supply abnormality and when update of the meta data of a hard disk is incomplete, the log data corresponding to the meta data for the update is held in the NVRAM, so it becomes possible to restore the integrity of the meta data on the hard disk by using this log data.