A stock solution is prepared by mixing and swelling TAC as a solute in a solvent, and is fed into an extrusion machine. The extrusion machine is cooled by a cooling medium, and is provided with twin-shaft screws. The screws are a compressing type where thread pitches of a helical flight decrease toward downstream. The screws are in mesh with each other and rotated in the same direction. Through the extrusion machine, the solute is dissolved in the solvent to form a dope. The dope is further cooled in a cooler to promote the solubility of the dope. Thereafter, the dope is heated by a heater to raise the fluidity. From the obtained dope, TAC film with excellent optical properties may be formed by a solution casting method.