The present invention provides a log file protection system which makes it difficult for computer log information to be altered or deleted. A log information reception process includes receiving log information output from an original application process 210, and storing it in an internal cache 232 in a NIGELOG processing module 230. A hiding directory information gathering process scans a file system 300 to collect information on directories in which hiding is possible. This information constitutes a list of directories 233 in which hiding is possible, and is stored as a variable in the NIGELOG processing module 230. This variable is used by the NIGELOG process whenever files are generated or moved. An alteration detection and log information writing process detects alterations in log files, automatically restores these when alteration is detected, and periodically re-hides log files.