1) go to magento root then in shell/ directory
2) execute "php -f log.php status" (in my case Data size is 150 MB)
3) execute "php -f log.php clean"
4) execute again "php -f log.php status" for checking the cleaning action. (it should show a decreased size from 150 MB, but it doesn't.
Well I truncated manually all the tables inside the databases and checking again the status it is decreased to 164 kb. This is a bug, this file is not cleaning the log tables. By the way I already reported in Bug List.