I'm looking for more police procedural/investigation/detective shows, please recommend some~ 

These are the ones i watched and liked - https://mydramalist.com/list/3bgwKg74

Thank you!

Since you liked Unnatural, you might like Hippocrates no Chikai, which is also about forensic medicine and its importance.

Yami no Bansosha is a good investigation drama too and one I wholeheartedly recommend.

Bloody Monday
Boku Dake ga Inai Machi
Keizoku
You Are Under Arrest
Strawberry Night+ Strawberry Night Saga
Ataru
Mr. Hiiragi's Homeroom
Akutou Juuhanzai Sousakan
Atami no Sousakan
Bitter Blood
Boku-tachi ga Yarimashita
Chinmoku Hotei
Death Note (2015)
Designer Baby
Final Cut
Hi wa Mata Noboru
Hidarime Tantei Eye
Hokuto
Lucky Seven
Marks no Yama
On: Special Agent Todo Hinako
Ouroboros: Kono Ai Koso, Seigi
Rakuen
Remote
Crminologist Himura and Mystery Writer Arisugawa
Sukeban Deka
Tantei Gakuen Q
Tokyo Dogs

Hero

The main character is a prosecutor, but he also spends a significant amount of time investigating crimes.

MIU 404

Surprised this hasn't already been mentioned.