A federal grand jury here is expected to question Rep. Edward J. Derwinski (R-Ill.) next week on charges that he tipped off the South Korean government when a Korean agent was about to take asylum in the United States.

Federal officials confirmed uesterday that a month-old investigation into who leaked the news of the impending defection has focused on Derwinski, a 19-year House veteran who is the ranking minority member of the Subcommittee on International Organizations.

Derwinski said yesterday that he expected to appear before the grand jury next week, but that he was not responsible for the leak.

"I'm not the culprit in this thing," Derwinski told reporters. "I'm assuming because of my consistent support for South Korea, and my skirmishes within the committee, I wind up as suspect No. 1 because of guilty by association."

According to The Wall Street Journal, which broke the news of the investigation yesterday, Justice Deaprtment officials believe that Derwinski warned South Korean officials last month when he learned that the chief new York agent of the Korean Central Intelligence Agency was about to break with the Seoul regime.

The Journal said the agent, Sohn Young Ho, who was officially employed as a diplomat in Korea's consular office in New York, approached the staff of the International Organizations Subcommittee on Sept. 15 and offered to cooperate with the subcommittee's investigation of KCIA operations in return for a grant of asylum.

The staff asked the FBI that day to begin protecting Sohn, the Journal said. But the Korean government also learned of Sohn's plan, and dispatched agents of its own to Sohn's New Jersey home the next morning. The Korean agents arrived just 30 minutes after the FBI had spirited Sohn and his family away, the Journal said.

According to Rep. Clement J. Zablocki (D-Wis.), chairman of the full International Relations Committee, the Justice Department immediately began searching for the source of the leak to Korea. The investigation centered on the members and staff of Derwinski's subcommittee, Zablocki said.

The case was brought to a grand jury here under a federal statute that prohibits effrots to "influence, intimidate or impede" a witness in a congressional investigation. The law carries a maximum penalty of five years in prison and a $5,000 fine.

Derwinski said yesterday that the subcommittee staff had told all members of the impending defection on Sept. 15, the day Sohn approached the staff. Derwinski said that he was never told Sohn's name.

Rep. Donald M. Fraser (D-Minn.), the subcommittee chairman, said yesterday that he did not think the leak to the South Korean government was accidental. Fraser refused to comment further.

No one familiar with the case would say yesterday how the investigation came to focus on Derwinski - except Derwinski himself, who said he was the suspect because os his strong support of the Seoul regime.

The congressman, an outspoken conservative on foreign affairs, is one of the most energetic anti-Communists in Congress. He is a harsh critic of Communist governments and a forceful supporter of foreign governments that are hostile to communism.

For years Derwinski has been a warm friend of both South Korea and Taiwan. He has defended those governments when they were accused in subcommittee hearings of human rights violations.

For more than a year, the subcommittee has been probing the activities of the KCIA in the United States. Staff members say that Derwinski has urged the Korean Embassy here to send official witnesses to present the government's position at the hearings.

Derwinski said yesterday there was no secret about his briendship with the Korean government. He told reporters he has had "regular contacts, almost routine" with Korean officials in Washington.

Derwinski's name has never come up in the investigations of the Korean influence-buying scandal on Capitol Hill.

He has received an honorary degree from a Korean unicersity and the Korean government's Gwanghwa Medal (for "diplomatic service merit"), but says he has never been offered valuable gifts or cash by any Koreans.

The subcommittee's investigation of teh KCIA has reviewed the agency's connections with the Rev. Sun Myung Moon, leader of the Unification Church, and its surveillance of Korean dissidents livving in the United States.

Sohn, who was allegedly the chief KCIA agent in New York, could presumably provide information about the Moon connection and about KCIA activities in the relatively large Korean-American community in the New York suburbs.

Sohn apparently knows little about the influence-buying program, however. Justice Department investigators questioned him at length after his defection and reportedly received little useful information on the scheme.