The Mole Song: Undercover Agent Reiji(2013)

Overview:

Reiji Kikukawa, who has a strong sense of justice, graduated from the police academy with the lowest score ever. He becomes a police constable, but is suddenly fired by the Police Chief due to "disciplinary" issues. In actuality, the firing is part of a carefully orchestrated plan to set him up as an undercover cop in the Yakuza.