Mattaku Saikin no Tantei to Kitara

Behold, a great detective who thrills the world solving impossible cases in no time! His name? Keiichiro Nagumo. …But all of that was over a decade ago. Now he's just your typical middle-aged dude. Nagumo is behind the times to the point that he doesn't even know what a smartphone is. But out of the blue, he is confronted by a high school girl named Mashiro who shows up wanting to be his assistant. Witness the miracles a gender-gap detective duo can make!

(Source: One Peace Books)