A college student buys a new apartment dirt cheap and the reason was that once the front door is opened from the inside it connect to a dungeon in another world instead so he goes to an adventure in the dungeon and meet new companions which treat him as a the Great Sage.

[Mangadex]
Does a two-bedroom apartment in the Tokyo metropolitan area with a rent of 30,000 yen sound too good to be true? Our hero certainly didn't read the fine print. After renting his dream apartment, Tooru quickly realized that the front door of his new home connects to a dungeon. But it isn't all that bad - after he saves a young knight, he quickly learns that his home can also be a rest stop for adventurers.