Actor Jang Keun-seuk began his mandatory military service on Monday, his agency Tree J said in the morning.

Earlier this month, the agency said the actor would do alternative service as a social office worker following his diagnosis of bipolar disorder in 2011, which classified him as ineligible for active duty in the military.

The agency didn't disclose where the 31-year-old actor will work for the coming two years.

Under the relevant law, he will go through educational programs run by the Military Manpower Administration before being assigned to a workplace.

On July 6, the actor posted a letter on his official fan page.

"I've been calmly waiting for this time to come ... I want to spend the coming two years more meaningfully than any event in my life has been for me," he wrote.

 "Since I debuted as a kid, I've neither taken enough rest nor had time for myself ... Remember, this is a short break. Take care of yourself everyone. I love you all." (Yonhap)