Arun Singh, bodyguard of Dhanbad Sadar MLA Raj Sinha, shot himself in his head from his service revolver at around 7.30 AM, police said.
He was immediately rushed to BCCL Central Hospital where doctors declared him brought dead.
Deputy Superintendent of Police (law and order) Amit Kumar, who rushed to the spot along with the MLA confirmed that the constable had shot himself.
The reason behind the action is being probed, he said.
Singh hailed from Buxar district of Bihar. He was posted as Sinha's security personnel in December last year.