Houkago no Dungeon Diver: Nippon to Isekai o Ikiki Dekiru You ni Natta Boku wa Level Up ni Isoshimimasu

Why not unwind in another world once in a while? High schooler Kudo Akira happens upon the ability to travel between Japan and the fantastical world of Do-Melta. Not bound by any dire mission or duty, he leisurely enjoys exploring his way through the massive dungeon in the center of the world and all the trappings that come with it. The guild receptionist is trying to mooch off of him, his magic instructor has "rewards" for him, and he just might have some fun with a girl he saves...

(Source: J-Novel Club)

*Note: Includes two extra chapters.*