The present invention relates to a method for activating protein wherein protein produced in a biologically inactive form (non-natural-form protein) is converted into a biologically active protein (natural-form protein) by bringing it into contact with cultured cells of an organism, and according to the present invention, the non-natural-form protein can be converted efficiently into the natural-form protein having activity, so the yield of the natural-form protein can be further raised by subjecting, e.g., culture of transformant to the activation treatment.