Saitei Chingin Maou: Rekidai Saikyou no Maougun, Sono Ashi wo Hipparu no ga Oshigoto desu

Arquas is a man who lives only for his hobbies and training. He has been content with his modest life as a bronze-rank adventurer, the lowest-class. One day, however, the goddess of creation, Willas, suddenly appears in his home and offers him a job—to become the Demon King and stand in the way of the strongest demon king's army in history. The only compensation offered is minimum wage, but even so, Arquas accepts the job.

(Source: Nihon Bungeisha, translated)