Working Dungeon! Shinsotsu Yuusha no Isekai Gyoumu Nisshi

A fresh university graduate, Sosuke Yatsuragi, finally lands his dream job at Nexus Pharmaceuticals, the very company that saved his life when he was a child. However, he finds himself assigned to the Materials Department, Division 2—a place notorious within the company as the "Human Graveyard," where redundant employees are sent to be phased out.

Early in the morning on his first day, Sosuke arrives at the office only to find a woman lying naked on the floor. Panicked, he rushes to help her, only to discover she is his new boss: Manager Miyu Yakushiji. When he learns the reason she was stripped bare was simply a habit of undressing while blackout drunk, Sosuke begins to fear he's been posted to the worst possible department.

His on-the-job training begins immediately, but something feels off. He is forced to change into a strange uniform and ushered onto a freight elevator without any explanation. The elevator descends further and further, passing floors that shouldn't exist. When the doors finally slide open, an unbelievable sight unfolds before his eyes...

(Source: AlphaPolis, translated)