Boku no Heya ga Dungeon no Kyuukeijo ni Natteshimatta Ken

There has to be a catch when a deluxe rental property is this cheap–and this one connects to a perilous dungeon in another world! Far from being dissuaded, Touru jumps at the chance to sign the lease, hoping to go from high school dropout to big-time adventurer. He soon encounters a stranded knight: a gorgeous young woman named Ria. Touru rescues her and takes her back to Earth, where she promptly mistakes him for a Great Sage, seeing normal things like running water for evidence of his powerful magic. Touru may dream of heroism, but first he has to figure out how to share an apartment with a cute girl from another world!

(Source: Seven Seas Entertainment)