Patent ID: 8361354

Claim:
A method of manufacturing an optical film, the method comprising: extruding a melted substance comprising a thermoplastic resin and irregularly shaped particles on a support from a casting die to form the optical film; wherein the casting die is equipped with, in order from an upstream side of an extruding direction of the melted substance: an inlet portion through which the melted substance flows in; a manifold portion; and a slit portion comprising an extending portion and a land portion, the melted substance being ejected from the land portion; wherein cross-sectional shapes of the extending portion satisfy the following equation (1) and cross-sectional shapes of the land portion satisfy the following equation (2): A 1 <A 2 (1) wherein A1 represents a cross-sectional area at an arbitrary position P 1 in the extending portion and A2 represents a cross-sectional area at a position P 2 which is upstream of the extruding direction of the melted substance in the extending portion, and A 3 =A 4 (2) wherein A3 and A4 represent cross-sectional areas at arbitrary positions P 3 and P 4 , respectively, in the land portion, positions P 3 and P 4 being different from each other.