Seikatsu Mahoutsukai no Gekokujou

I am not “useless”! I'll prove it by conquering dungeons!

One day, magical dungeons suddenly appear in modern Japan, and life is completely transformed. Midorimu Sakaki was born with a talent for life magic, which doesn't help with dungeon exploration or defeating monsters. So even though his power level is the highest rank S, he was nevertheless scorned as useless and could only find work as a clerk at a magic academy. But by chance, he comes into possession of a rare item, “Sage System,” which allows him to create new types of magic during dungeon exploration. Now he can use the system to create life magic suitable for dungeon exploration, and finally take his first step to fulfill his dream as a dungeon adventurer!

(Source: Kadokawa, translated)