The invention relates to a purification unit (200) which is able to separate crude MMA from light and heavy impurities in order to obtain high quality of MMA, suitable to produce optimal grade polymethylmethacrylate (PMMA). The unit (200) comprises two distillation columns (210, 250) in series, fed with mixture to be distilled in their median part, in order to separate each column in two upper (213, 253) and lower (212, 252) parts, the first distillation column (210) being fed with crude prewashed MMA, and the second distillation column (250) being fed with distilled liquid stream containing MMA, separated from light impurities, issued from bottom of first distillation column (210). The upper part (213) of first distillation column (210) is connected to a lateral extraction system (220), able to minimize MMA content in light impurities flowing upward said first column (210).