lol, ok. This is the quest under Black Paper 1. In Japanese, yes. lol
To do it however, you need a Photon Crystal. (Found only on ep 4).
When you get one, start the quest, it has to be in one person mode, under side quests.
Exit the quest room after starting it, and there will be a blond Ranger at the corner. Just keep pressing enter and he'll take you underground. Then talk to the girl.
She'll ask a few questions, which you can't read. Just keep answering them till the door unlocks. Then kill everything. You'll know when the quest is over because the signature door unlocking sound will occur.
Enter the door, and there will be an item in front of the portal.