Provided is a method capable of producing a protein at a high level using a cultured animal cell, comprising culturing a cell that expresses APES (Antibody Production Enhancing Sequence) and into which a DNA encoding a desired polypeptide has been introduced, thereby producing the desired polypeptide. APES contains a nucleotide sequence related to nuclear factor κB inhibitor α (NfkBia) and has a function of decreasing the intracellular expression of NfkBia.