A method for reducing the formation of a byproduct polypeptide containing an O-acetylserine residue in place of a serine residue by adding at least one of histidine, methionine or glycine to the medium in a method for producing a polypeptide containing a serine residue by culturing transformed cells, and a method for producing a polypeptide containing a serine residue by culturing transformed cells, characterized by reducing the formation of a byproduct polypeptide containing an O-acetylserine residue in place of a serine residue by adding at least one of histidine, methionine or glycine to the medium.