A process for producing lactic acid according to the invention is characterized in that glycerin is subjected to a hydrothermal reaction under an alkaline condition at a temperature in the range of 150 to 400° C. and under pressure equal to or more than the saturated vapor pressure at the temperature. The glycerin produced from plant fats, animal fats or the like or pure product synthesized chemically or a discharge containing glycerin generated at the production of diesel fuel oil from fats, in which the fats are subjected to a transesterification with alcohol in the presence of an alkali catalyst in order to obtain fatty acid ester is preferably used as a starting material.