I almost fixed
Hmmm.... I think another program in a hosting server is already using POP3 class somewhere.
I changed the class name POP3 to POP33. Also changed "class.pop3.php" to "class.pop33.php".
In addition, I changed a instance for "pop3" to "pop33" at line 192 as well as I change to "require_once(INCLUDE_DIR.'class.pop33.php');" in admin.php.
After that, I can access to Admin Panel, but I think php3 function is not working correctly.
Please tell me which part I was wrong.
Thanks you.