The investigation of a controversial South Korean professor who makes no secret of his pro-North Korean sentiments is stimulating debate over the country's National Security Law. The law prohibits praising or assisting the North's communist regime in any way, but President Roh Moo-hyun and his political allies say it is obsolete and should be revoked.

South Korean sociology professor Kang Jeong-koo has some controversial views about the 1950s war between North and South Korea.

"The United States has no right to intervene in internal affairs of the Korean peninsula," said professor Kang. "Because of the intervention of the United States, the Korean peninsula, the Korean people as a whole, had to suffer."

Professor Kang says that if the United States had not intervened to defend South Korea after the North invaded in 1950, Korea would have long since been unified under North Korean socialism, an outcome he believes most Koreans wanted at the time.

Earlier this year, professor Kang signed a guest book in North Korea with a message calling for unification that cherishes "the spirit of Mangyeongdae." Mangyeongdae is the reputed birthplace of North Korea's first leader, Kim Il Sung, and the spot where his embalmed body lies on permanent public display.

Professor Kang's expressions of sympathy toward North Korea are more than controversial. They could land him in prison. He is being investigated by the police as a possible violator of South Korea's National Security Law.

The 1988 measure, like a previous version, forbids "anti-state activities" - a reference to praising, assisting, or collaborating in any way with the Pyongyang government. It has severely restricted South Korean's access to North Korean representatives and media.

Supporters of the law say its necessity derives from North Korea's long history of destructive espionage in the South. President Roh Moo-hyun argues the law was often used as a tool of repression by previous authoritarian governments. He is supported in that view by several major human rights organizations, including Amnesty International.

Mr. Roh and his Uri party want to scrap the law, while the conservative opposition aims to keep it, or at least an amended version of it, on the books.

Kang Jae-sup, the floor leader of South Korea's main opposition party, the conservative Grand National Party, is harsh in his condemnation of professor Kang, to whom he is not related.

Mr. Kang says he believes professor Kang shouldn't even breathe the air of a democratic society.

Mr. Kang and other conservative lawmakers accuse President Roh's government of refusing to enforce the law adequately. They are trying to have South Korean Justice Minister Chun Jung-bae removed from office for ordering that Professor Kang not be held in custody during his investigation.

They also criticize the Seoul Unification Ministry's speedy approval of travel permits allowing hundreds of South Koreans to visit Pyongyang this month for the Arirang Festival, a giant North Korean propaganda event.

Supporters of President Roh's policy of cooperation and engagement with Pyongyang say expanding tourist and business contacts with the North renders the National Security Law obsolete. They say the gap between North Korea's withered economy and that of the South, the 11th largest in the world, proves that the ideological battle between the two countries has already been won.

Casual conversations in downtown Seoul show the split in public sentiment over the law, mainly along generational lines.

Lee Kyoung-ah, a woman in her 20s, says it is time to get rid of the law. Ms. Lee says in South Korea's free society, people should have access to North Korean media and make judgments for themselves.

Lee Tae-hyun, a man in his 50s, says it is still too early to repeal the law. Mr. Lee says despite thawing North-South relations, the two countries still stand in confrontation to one another. He says when the North-South relationship improves further, maybe then it will be time to talk about getting rid of the law.

Experts say the controversy over the National Security Law highlights South Korea's modern-day dilemma, confirming its transformation to a rights-oriented democracy while protecting itself from possible communist subversion.

A decision in professor Kang's case is not expected for several months.