PURPOSE: To obtain the titled film useful as a base for magnetic tapes, by bringing a polyester film having surface adhesivity into contact with a monocarboxylic acid and a monohydric alcohol on a heated roller at a constant temperature, eliminating the adhesivity.

CONSTITUTION: The polyester film 10 having experienced at least thermal history, showing adhesivity by the exposure of the carboxyl group and hydroxyl group contained in it to the surface of the film is fed from the delivery roller 11 to the heated roller 13 kept at a temperature satisfying both enough temperature to react the hydroxyl group on the surface of the film with a monocarboxylic acid W150°C, and enough temperature to react the carboxyl group on the surface with a monohydric alcohol W150°C in such a way that the surface of the film put outside, the monocarboxylic acid (especially acetic acid) 16 and the monohydric alcohol (especially ethanol) 19 are applied to the film by the applicators 17 and 18, respectively, brought into contact with it, and the hydroxyl group and the carboxyl group are converted into corresponding esters so that adhesivity is eliminated.