Optical film, optical film manufacturing method and optical film manufacturing device

An optical film producing method, comprising steps of:

• extruding a melted material including a melted cellulose ester resin from a casting die by using an extruding device;
• forming a film by pressing the extruded melted material between a first rotating member and a second rotating member;
• and
• winding up the formed film,
• wherein the following formulas are satisfied:
• 30° C.≦T1−T2≦60° C.
• 0° C.≦T2−T3≦20° C.
• Tg<T3
• where T1 is the temperature on an outer surface of the melted material at an exit of the casting die, T2 is the temperature on an outer surface of the film at a point where the film firstly contacts the first rotating member, T3 is the temperature measured on an outer surface of the film at a point where the film is pressed between the first rotating member and the second rotating member by separating the first rotating member from the film and thereafter measuring T3, and Tg is the glass transition temperature of the film and wherein the outer surface is a surface being not in contact with the casting die or the first rotating member.