Patent ID: 7785503

Claim:
A production method of an optical film composed of a thermoplastic resin film, including a step of cooling a molten thermoplastic resin extruded from an extruding machine by making it successively circumscribed with a first cooling drum, a second cooling drum and a third cooling drum: wherein, a temperature difference of said first cooling drum and said second cooling drum is made to be 10° C. or less to cool said thermoplastic resin, when, assuming a resin contact time in said first cooling drum is t 1 (sec.), a temperature when said thermoplastic resin moves away from said first cooling drum is Tp 1 (° C.), and a glass transition temperature of said thermoplastic resin is Tg (° C.), the relationship: t 1 ×(Tp 1 −Tg) (unit: sec.·deg) is made to be −50 or higher and 20 or lower to cool said thermoplastic resin, and said thermoplastic resin film satisfies a relation of the formula: [sin 2 2α]×[sin 2 (π· Re/ 550)]≦3.4×10 −5 over the whole surface of the film when an angle made by the extruding direction of the thermoplastic resin film from the melt extruding machine and a slow phase axis at each point is a and a retardation amount at each point is Re.