TOKYO, JUNE 2 -- The planned succession of ruling party leader and retired general Roh Tae Woo as the next president of South Korea moved a major step closer to reality tonight, as President Chun Doo Hwan publicly endorsed Roh for the first time.

Chun's choice, announced by a ruling party spokesman, had been known unofficially for months. It makes nomination of Roh for president at a party convention June 10 a foregone conclusion.

Roh will run in an election late this year, which the main opposition party has said it will boycott. He is seen in Seoul as virtually certain to be declared winner and inaugurated for a seven-year term.

Chun has stressed that his departure from office next year will be the first peaceful transfer of power in South Korean history.

Roh is one of the few people with close personal ties to Chun, who has ruled with an imperial aloofness. The two men graduated from the 1955 class of the South Korean military academy and later served together in South Vietnam.

After the 1979 assassination of president Park Chung Hee, Roh brought troops under his command into Seoul to support a successful bid for power by Chun.

Roh has since served in Chun's Cabinet and as chairman of the Seoul Olympic Organizing Committee, which is preparing for the 1988 Summer Games to be held in the South Korean capital. He is now chairman of the ruling Democratic Justice Party.

Chun tonight called Roh and about 25 members of the ruling party's executive committee to the Blue House, his official residence, and told them formally he wants Roh to be the candidate, party spokesman Kim Chung Nam said.

Presidential spokesman Lee Jong Ryool said Roh was chosen after party leaders agreed that the next president should be a man with a good understanding of the Korean Peninsula's security problems and wide knowledge of government.

Roh was quoted as saying he would accept Chun's decision as a "historic mandate."

The main opposition Reunification Democratic Party sees the coming presidential election as substituting one military man for another and says it will bring no significant change to the political system here.

Analysts in Seoul expect Roh will follow the same basic policies as Chun. However, he is more outgoing than Chun and has wide experience as a political operator, dealing with opposition leaders face-to-face on a regular basis.

The opposition party, along with groups of clergymen, students and workers, plans mass demonstrations, a traditional form of protest, on the day of the ruling party's conventions.

The election is to be conducted under an indirect voting system, in which about 5,000 elected members of an electoral college will choose the president in closed session. The opposition says the system is open to manipulation by the government.