As a part of the development of process technology related to the manufacture of polylactic acid, it is important to obtain data on the correlation of the moisture content ratio and water vapor partial pressure with respect to lactic acid concentration. To eliminate the effects of the mixing of lactic acid with the water vapor, we have tested the combination of a water vapor transmitting filter and an Othmer equilibrium still, and determined the following. 1) For heating temperatures of 135 degrees C and 150 degrees C, the moisture content ratio in the lactic acid is reduced to 2% or less in two hours. After that, however, the decrease becomes gradual, so it is reasonable to consider that as the stopping point. 2) The reason for the first result is that, for these heating temperatures, moisture evaporation maintains a constant partial pressure down to a moisture content of 2%, but the vapor pressure decreases after that point in accordance with Henry's law.