It's the first public sign of discord in the tightly choreographed succession process, though analysts said Kim Jong Nam spends so much time outside his native land that his opinion carries little weight.
The chubby 39-year-old Kim, the oldest of three brothers who were in the running to take over secretive North Korea, is the closest thing the country has to a playboy.
Unlike many of his countrymen back home who lack the resources and connections to travel overseas, Kim travels freely and spends much of his time in China or the country's special autonomous region of Macau - the center of Asian gambling with its Las Vegas-style casinos.
He sports the family pot belly and favors newsboy caps and an unshaven face, while frequenting five-star hotels and expensive restaurants. In June, he was photographed in Macau wearing blue Ferragamo loafers.
Speaking in Korean, he told Japan's TV Asahi, in an interview from Beijing aired late Monday and Tuesday, that he is "against third-generation succession," but added, "I think there were internal factors. If there were internal factors, (we) should abide by them."
"I have no regrets about it. I wasn't interested in it and I don't care," Kim said, when asked whether he is OK with the succession plan.
Kim said he hopes his brother will do his best to bring abundance to the lives of North Koreans and that he stands ready to help from abroad, according to a dubbed Japanese-language version of his remarks.
Kim Jong Un, believed to be 26, appeared with his father at Pyongyang celebrations on Sunday marking the 65th anniversary of the ruling Workers' Party, saluting troops marching past in a massive military parade and waving to the crowd. The appearance was less than two weeks after he was named to a top political post and promoted to four-star general.
Andrei Lankov, a Russian expert on North Korea at Seoul's Kookmin University, said Kim Jong Nam's remarks were "almost a challenge," but noted he has little influence due to the considerable time he spends abroad and lacks military support.
"I don't see them rallying to Kim Jong Nam," he added, emphasizing that key generals who run the military far prefer Kim Jong Un, who they see as young, inexperienced and thus easy to control.
Kim Jong Il is known to have three sons - one from his second wife and two from his third. He favors his youngest, Jong Un, who looks and is said to act like his father, according to the leader's former sushi chef. He studied at a Swiss school and learned to speak English, German and French, news reports have said.
In contrast, Kim often derided the middle son, Jong Chul, as "girlish," the chef, who goes by the pen name Kenji Fujimoto, said in a 2003 memoir. Little is publicly known about the brother, except that he also studied in Switzerland and is a fan of U.S. professional basketball.
Jong Nam is widely believed to have fallen out of favor after embarrassing the government in 2001 by being caught trying to enter Japan on a fake passport, saying he wanted to visit Tokyo Disneyland.
Experts said Kim Jong Nam will most likely continue living abroad, with fewer reasons than ever to return to Pyongyang.
"In the future Kim Jong Nam will have little influence on the political situation in North Korea. It's very unlikely he will go back. His force within the country is now almost nonexistent," said Cai Jian, deputy director of the Center for Korean Studies in Shanghai's Fudan University.
Associated Press writers Kelly Olsen and Kwang-tae Kim in Seoul, Shino Yuasa in Tokyo and researcher Xi Yue in Beijing contributed to this report.