Isekai Kenja no Mazoku Hanzai Chousa

Jitte Akutsu is a police detective who never does his job well, and always longed to be reborn into another world. Then, after getting wrapped up in a case, he winds up in critical condition and on the verge of death. But who appears before him? A sage with an obsession with mysteries and who, after hearing that Akutsu was a police detective, decides to give him "another chance" and sends him back to his world! Not only that, but the sage decides to follow Akutsu to his world and solve crimes together with him?! Source: Kodansha