Ore ga Isekai Dungeon da! – Tensei Shitara Karada ga Dungeon to Kashiteita

Ore ga Isekai Dungeon da! – Tensei Shitara Karada ga Dungeon to Kashiteita summary:

While on his way to work, white collar worker Shoutarou Sagami got hit by a truck and died. And then, he was reincarnated–into a dungeon.
Whatever fate it was that caused Shoutarou Sagami’s body to become a dungeon, he decided to cultivate himself as a dungeon.
A story where a guy reincarnated with the body of a dungeon decides to advance of his own will. He starts as just a cave, and aims to become a gigantic labyrinth. But before he can get that far, he shelters a lost Elf, supports the cat-eared merchants in front of his entrance, and much more.