Abstract: PROBLEM There is provided a method for producing polylactic acid, which is capable of obtaining polylactic acid at low cost, and having high molecular weight. SOLUTION A method for producing polylactic acid by polymerizing molten lactide using at least one reactor having plug flow characteristics with ?(?), calculated from the following FORMULA (1), of 0.3 or smaller: ?(?)=?0?(E(?)×ABS(1??))d???(1) in the Formula (1), E(?) is a residence time distribution function determined by impulse response by starch syrup having a viscosity of 3 Pa·s, and ? is ratio of elapsed time ? and mean residence time ?0.