Moon Jae-in, former chief of the biggest opposition Minjoo Party, kept his top post in recent opinion polls for a race to replace ousted President Park Geun-hye.
According to a Realmeter survey released Thursday, Moon gained 37.1 percent in approval rating as next leader. It was up 2.0 percentage points from the previous week.
The results are based on a poll of 1,015 voters conducted Wednesday. It has 3.1 percentage points in a margin of error.
The country entered a presidential race as the constitutional court upheld the motion to impeach Park on March 10. A presidential election to succeed Park was set on May 9.
Prime Minister Hwang Kyo-ahn, who has served as acting president, on Wednesday declared his drop in run for president but there was no dramatic change in opinion poll rankings among presidential contenders.
Hwang had been viewed as the best hope in the conservative bloc, which fell together with Park's downfall over a corruption scandal that led to Park's impeachment and ouster.
Before his Wednesday declaration, Hwang came next to Moon in presidential polls though there had been a wide difference.
Governor of South Chungcheong province Ahn Hee-jung of the Minjoo Party ranked second with 16.8 percent in approval scores, up 2.7 percentage points from a week earlier.
Ahn seemed to have benefitted most from Hwang's drop in the presidential bid as he ranked first in opinion polls in the North Gyeongsang province, a traditional support base for Park and conservative politicians in the past.
The governor recently gained support from conservative voters for his conservative remarks, but he lost much of liberal support for the same comments. It means he may get less competitive in the primary of the leftist-leaning Minjoo Party.
Ahn Cheol-soo, former chief of the minor opposition People's Party, moved to the third place with 12 percent in support scores. He was followed by Seongnam city mayor Lee Jae-myung of the Minjoo Party with 10.3 percent.
Meanwhile, Hong Jun-pyo of the Liberty Korea Party emerged as the best hope to replace the prime minister as a presidential candidate in the conservative camp.
The governor of South Gyeongsang province, another political support base for conservative voters, garnered 7.1 percent in support scores, almost doubling 3.6 percent tallied in the previous week.