Nozomanu Fushi no Boukensha

From J-Novel:
It's been ten years since Rentt Faina, a down-on-his luck Bronze-class adventurer, set his eyes on becoming Mithril-class. Unfortunately, Rentt finds himself helpless when confronted by a legendary Dragon in the Labyrinth of the Moon's Reflection and is summarily eaten. But Rentt has miraculously reawakened as a Skeleton?

Original Manga