Speculation has recently grown again that Kim, who is 66 and has not been seen in public for more than three weeks, is unwell. Some media have long thought that Kim, a former smoker and heavy drinker, was ill but Seoul intelligence officials say they believe he has diabetes and heart problems, but those are not serious enough to affect his job.
But a book by Japan's Professor Toshimitsu Shigemura at Japan's respected Waseda University says Kim died in the autumn of 2003 and a series of stand-ins have since taken his place at official state event.
Prof Shigemura says Kim was not seen in public for the 42 days after September 10, 2003, and in his book "The True Character of Kim Jong Il" claims the man that North Koreans refer to as the "Dear Leader" died of diabetes.
"In the years before he died, Kim took some really big decisions on North Korea's relationships with the outside world," says the professor, pointing to the historic June 2000 summit with South Korean President Kim Dae Jung, a visit from Russian leader Vladimir Putin the following month and then US Secretary of State Madeleine Albright in October 2000.
The following January he was in China, met Japanese Prime Minister Junichiro Koizumi in September 2002 - and admitted that Pyongyang had abducted Japanese nationals to train its spies - and August 2003 saw the opening of six-way talks on halting North Korea's nuclear weapons programmes.
Then, suddenly, Kim disappeared, says Shigemura, and there was chaos in the upper echelons of the country's leadership. "I have been working on the book for four years," said Shigemura, a former journalist for the Mainichi newspaper who was posted to Seoul for six years from 1979 and then served for another five years in Washington D.C. A North Korean agent told him in 1995 that he had met one of Kim's doubles - there have been as many as four - and that he used them to stand in at outside ceremonies because he was fearful of a coup.
After Kim's death, a group of four very senior officials in the regime decided to protect their own positions by making the stand-in more permanent. Whenever anyone meets the North Korean leader, Shigemura says one of the four is alongside him "like a puppet-master."