Vampire Prosecutor (2011)

both have supernatural side to the drama and it both includes investigation for the truth ^^

They both involve investigation..they both have small scale cases to solve which is associated with an on going big one.
Both leads have some kind of special ability...
Difference~
One has super natural elements, the other doesn't.

Both Tunnel and Vampire Prosecutor are investigative crime dramas. They both have intense cases each episode, and also a good sense of humor and camaraderie between the investigation teams.

both have vamps with good conscience

Yamihara Kokoro and Asada Toramatsu live in a town that is quite peaceful, until a serial murder case happened where the bodies all had their blood drained out. Five years have passed but the case remains unsolved. Meanwhile, Kokoro and Toramatsu have been in a relationship for a year, but are unable to get married as they are keeping a serious secret from each other. Kokoro is actually a vampire. However, she does not attack humans and live modestly. Toramatsu is a popular resident in the town and is trusted by many. After getting a divorce, Toramatsu got drunk and went back to his old house, where he met Kokoro, the current occupant. As the two continue to keep their secrets hidden, a horrific murder occurs once again in the town.

Both shows are really dark, and they both have a sexy male lead who´re very charismatic and with a dark past.