Kim Hong-gul's questioning has caused a media frenzy
He is suspected of taking hundreds of thousands of dollars in bribes in return for using his influence to help a business firm obtain a lucrative lottery licence.
Arriving at the Seoul district prosecutor's office with his lawyer on Thursday, Mr Kim is likely to face several days of questioning.
Apology
Television stations broke off from regular programs to cover Mr Kim's tearful apology live before he entered the prosecutor's office:
"I am sorry. I feel ashamed before my parents and apologise to the people," he said.
Kim Hong-gul is accused of using his influence to help Tiger Pools International win the nation's first sports lottery business license in February last year.
Another son of President Kim, Hong-up, is also suspected of involvement in a separate influence-peddling case and may be summoned soon for questioning over a scandal in which powerful politicians were allegedly bribed to cover up business wrongdoing.
Damaging scandals
The cases have created a huge political storm, with the unions and the opposition Grand National Party (GNP) threatening political and labour unrest in protest during the World Cup, which South Korea hosts in a few weeks' time.
President Kim apologised to the nation for the scandal concerning his sons earlier this month and resigned from his party to concentrate on state affairs, but the country is apparently not placated.
The popularity of President Kim, who was elected on promises of reform, has plummeted. One poll put his approval rating at just 16%.
Although the president is constitutionally barred from seeking a second term in office anyway, the series of corruption scandals surrounding his aides, relatives and government officials have damaged the ruling party.
The approval ratings have dipped for the Millennium Democratic Party's new reformist candidate for end of year presidential elections, and are also likely to affect the MDC's chances in local elections in June.
Deja vu
South Korea is seeing history repeating itself.
Five years ago on Wednesday, Kim Hyun-chul, the youngest son of former President Kim Young-sam, who was the same age as Kim Hong-gul at the time, was summoned to the same prosecutor's office to answer charges of corruption and influence-peddling.
Mr Hyun-chul, who earned the nickname of Mr President Junior because of his influence over the administration, was later sentenced to two years in prison on charges of taking $6.6bn in bribes, and evading tax.
That scandal, and the ensuing financial crisis, helped Kim Dae-jung win the 1997 presidential election.