My Room is a Dungeon Rest Stop (Manga), Vol. 3

Touru's Tokyo apartment may lead to a dungeon in another world, but that doesn't mean he's strong enough to defeat the monsters lurking within it. So, through the power of modern science, Touru comes up with a risk-free Level-Up Plan, hoping to extend his life and become the Dungeon Master. But has someone else besides Touru made their way into the other world?!