Sorry guys, the loader.sin in the package is incorrect. Here is how to fix it.
1. Download correct loader: http://www.multiupload.com/KN7SQNKLQ2
2. Open LT15i_3.0.D.2.79_DoCoMo_JP.ftf with WinRAR
3. Copy and replace loader.sin in the .ftf fil...
Basically Docomo says this model cannot be UNLOCKED and this has to keep specified ROM.
But finally I could UNLOCK by flashing global ROM and after completing ROM flashing, unlock screen appeared. Therefore I could enter unlock code successfully!...