PURPOSE: To make it possible to obtain the latest log data securely and quickly when an system error occurred, by outputting positional information showing usually the latest and its effective log area in external log file.

CONSTITUTION: At first, a log data from information processing system is stored in order with byte unit at a block of a log buffer 13 through a log record 12. Next, processer seeks for final address of stored log data and stores it in a final position storage part 15. Whenever the log is stored buffer 13, it is outputted the block corresponding to one in a log file 21 of storage 20. At that time, the final positional information is outputted with the log data. Thus, the latest log data and, head positional information and final positional information showing the effective log area of the latest log data, are stored in contents of the log file 21. Namely, when the system error occurred, the latest data is obtained from file the file 21 securely and quickly, and this makes it possible to clear up the cause of the system error.