The benchmark Korea Composite Stock Price Index lost 9.47 points, or 0.40 percent, to 2,379.24 as of 11:15 a.m.
Kim Ye-eun, an analyst at Cape Investment & Securities, said institutions sold large-cap stocks in apparent profit-taking, which are to blame for losses on the main bourse.
Large-cap stocks were mixed across the board.
Market bellwether Samsung Electronics Co. gained 0.19 percent, and No. 2 chipmaker SK hynix Inc. was up 2.29 percent.
Meanwhile, top automaker Hyundai Motor fell 0.35 percent and Naver, the operator of the country's top Internet portal, lost 1.31 percent.
The local currency was trading at 1,130.95 won against the US dollar, up 5.55 won from the previous session's close.