The State Department's senior Asia expert and a leading liberal Democratic member of Congress agreed yesterday that, based on current information, election irregularities in South Korea could not have significantly affected the decisive victory of Roh Tae Woo as the country's next president.

"We have no evidence of systematic fraud in this election," said Assistant Secretary of State Gaston J. Sigur, testifying before the House Foreign Affairs subcommittee on Asian and Pacific affairs.

"There are instances of irregularities," Sigur said. "As far as whether the election would have come out differently if these irregularities had not taken place, every reading that we have at this time would indicate no. And that has come from even certain groups which are clearly antigovernment in Korea."

Subcommittee Chairman Stephen J. Solarz (D-N.Y.) said that on the basis of Sigur's testimony and other available information, including Roh's margin of about 2 million votes, "It is very hard to understand how the results could have been affected by massive and systematic fraud."

In what appeared to be an appeal to opposition leaders, who have been on friendly terms with Solarz in the past, the New York lawmaker said that "those people who have been actively involved in the struggle for democracy in South Korea, who have looked to the United States and particularly to the Congress for at least moral support in their struggle to achieve the same kind of democratic system in their country that we have in ours, have to recognize that precisely because of our commitment to democracy we feel obligated to respect the results of an election that fairly represents the will of the people."

Solarz added, "If they expect us not to accept those results, they have to produce convincing evidence that the results were not freely and fairly arrived at."

Rep. Jim Leach (Iowa), the subcommittee's ranking Republican, said there are ironies in the circumstance that "the opposition that fought so hard for direct elections found themselves defeated because of direct elections." A major reason for this, he suggested, was the failure of the opposition to unite behind a single candidate, leading to an almost even split in the vote between Kim Young Sam and Kim Dae Jung, rival opposition leaders.

Sigur said that most Koreans appeared to be against large-scale street demonstrations. He characterized the demonstrations since the election as limited in participation and arising from "somewhat extreme" elements.

President Reagan, in a congratulatory message to Roh announced at the White House, pledged to work closely with the new leader. Reagan said Roh "faced formidable tasks" in domestic and foreign affairs and said he will have the "full support" of the United States.