His appearance came on the heels of a rocket launch heralded by North Korea as "historic" but assailed elsewhere as provocative.
Limping ever so slightly as he arrived to a standing ovation from fellow deputies to the 687-seat Supreme People's Assembly, he returned their applause by clapping, then motioned for them to be seated, footage broadcast on state TV Thursday evening showed.
Legislators approved Kim as chairman of the powerful National Defense Commission, an appointment that under the constitution makes him the nation's top leader while his father, late North Korea founder Kim Il Sung, remains "eternal president."
"Having comrade Kim Jong Il at the highest post of our country again is a great honor and happiness for our military and people and a great happy event for all Korean people," a newscaster said on state TV.
The appearance may put to rest some questions about whether Kim, 67, has recuperated from a stroke that U.S. and South Korean officials say he suffered in mid-August.
His failure to attend a milestone parade marking the 60th anniversary of the country's founding in September had sparked concerns about his health and fears of a succession crisis in the nuclear-armed nation. Pyongyang denies he was ever ill.
Kim re-emerged in state media in early October and has steadily visited farms, factories and military units for "on-the-spot field guidance" in an energetic tour widely seen as an attempt to squelch rumors about his health.
However, recent photos showed him looking frail and extremely thin, having lost his burgeoning belly. Images released March 19 by the official Korean Central News Agency show Kim gripping the railings of a swimming pool in what looked to be an effort to hold himself upright.
On Thursday, Kim looked healthier than in the photos but the weight loss appeared to have been sudden, leaving the skin on his once-pudgy face hanging loosely.
North Korea has sought to build unity and support for Kim in the days preceding the opening session, first with Sunday's controversial rocket launch, then with a series of documentaries paying homage to his leadership.
North Koreans got their first glimpse Tuesday of the rogue regime's rocket launch, more than two days after a liftoff decried by the U.S., Japan, South Korea and other nations as a provocation that merits international censure.
North Korea claimed it successfully put a communications satellite into orbit and that it was transmitting data and playing patriotic odes to Kim and his father, the country's founder.
U.S. and South Korean military officials say nothing made it into orbit and accuse Pyongyang of using the launch to test its long-range missile technology. Washington, calling the launch a bold violation of U.N. Security Council resolutions barring North Korea from ballistic missile-related activity, is leading the push for council condemnation.
However, council debate remains stalled, with North Korea's closest ally, China, and Russia maintaining calls for restraint.
As the nation's top military chief, Kim rules under a policy of "songun" - or "military first" - overseeing a 1.19 million-strong armed forces that is one of the world's largest. Kim also is the top official in the powerful Workers' Party and supreme commander of the army.
North Korea's No. 2 leader, Kim Yong Nam, lauded the leader for building up the country's "invincible forces" and for the "successful" launch of the satellite Sunday. He also praised Kim Jong Il for "crushing the imperialists' policies to stifle" the North - an obvious reference to the United States.
Outside observers were watching Kim closely for clues to his health and any signs he may be laying the groundwork for a successor.
Kim has ruled the impoverished nation of 24 million with absolute authority since his father's death in 1994, allowing no dissent or opposition. The two Kims have thrived on an intense cult of personality, with their portraits hanging in nearly every room.
It's unclear whether the communist dynasty will extend to a third generation. None of Kim's three sons was elected to parliament in March, and they are not believed ready to assume the leadership mantle.
Despite the limp, Yang Moo-jin, a professor at Seoul's University of North Korean Studies, said it was clear "Kim Jong Il has no problem ruling the country."
There were no major changes in the lineups of the National Defense Commission, the Cabinet and parliament's presidium. Some Cabinet ministers, mostly economy-related posts overseeing such sectors as finance, commerce, electronic industry and fisheries, were replaced.
Associated Press writers Jae-soon Chang and Hyung-jin Kim contributed to this report.