In this study, the possibility of tri-n-decylamine (TDA) was investigated as the extractant in extractive lactic acid fermentation with a recombinant yeast being used. Extractive fermentation with TDA did not provide high l-lactic acid production relative to fermentation without extraction. Through determination of trace components of the TDA, it was found that the TDA contained a high concentration of 1-decylaldehyde, which was confirmed to be toxic to yeast and to have an inhibitory effect on cell growth even at low concentration (40ppm). When 1-decylaldehyde in TDA was reduced from 700ppm to 33ppm, the productivity and total concentration of lactic acid increased by 1.8 times and 2.5 times, respectively.