A method of purifying L-ascorbic acid which comprises: bringing an acidic aqueous solution of L-ascorbic acid which contains a cationic surfactant into contact with active carbon activated by chemicals.The method is in particular useful for removing a cationic surfactant from a reaction mixture of diacetone-2-keto-L-gulonic acid or 2-keto-L-gulonic acid with a mineral acid in the presence of an inert solvent and a cationic surfactant, to produce L-ascorbic acid.