Controller problem : cannot load it's index automatically

[eluser]anggie[/eluser]
Hi all...
I have search within the forum and didn't find answer, so i decided to ask you guys :
Here is my problem:

I have a controller named "Admin" and of course I give it an index() and _construct method.
Normally, if I call mysite.com/index.php/admin, it'll load index() method,right?

BUT IN MY ADMIN CONTROLLER NOT!

It ask me to add an index segment (mysite.com/index.php/admin/index) for real result.
I don't know where is my wrong, because i have another controller that act normally like.

Would you mind helping me? Thanks in advance.

sorry for my bad English.