An optical film characterized by a high degree of flatness that can be used as a phase difference film of a liquid crystal display, and a method for manufacturing this optical film according to melt-casting film forming method. A melt including a cellulose resin is extruded onto a rotary support member 5 (cooling roll) from a flow casting die 4. A rotary pinch member 6 (touch roll) is energized to the rotary support member 5, and a nip is formed between them. When the melt is sandwiched and pressed by the nip, a die line and irregularity in thickness are removed. The linear pressure of the rotary pinch member 6 and the temperature of the melt immediately before the melt is sandwiched and pressed by the nip are set within a predetermined range, whereby effective correction of the film flatness is achieved.