The present invention provides a method for alkaline saponification of a polymer film, comprising the steps of: alkaline saponification of the polymer film with an alkaline solution; and washing away the alkaline solution from the alkali-saponified polymer film, wherein the washing step includes a plurality of water-washing steps of washing the alkaline solution coated on the polymer film away using washing water, along the travel direction of the polymer film, and the used washing water is reused in order to perform alkaline saponification of a polymer film which allow efficient use of washing water and realize low cost and low environmental load while maintaining quality stability.