In a method for producing a lactic acid oligomer by subjecting an aqueous lactic acid solution to a polycondensation reaction under microwave irradiation, the polycondensation reaction is carried out under reduced pressure in the absence of a polymerization catalyst and an organic solvent. An initial polycondensation reaction is carried out under normal pressure, and thereafter, a reaction is carried out under reduced pressure. The pressure is reduced stepwise to give a final pressure of 2000 Pa or less. A lactic acid oligomer which has a high purity and is safe from hygiene point of view can be obtained at a low cost in a short time.