A method for manufacturing an optical film comprising the steps of: 1) casting a dope (a solution), prepared by dissolving thermoplastic resin film raw materials in a solvent, from a casting die onto an at least 1.8 m wide belt support, and 2) drying a cast web (a cast film) peeled from the belt support, wherein the belt support has a thickness (Te) at both right and left end portions, of 5-20% less than a thickness (T) of the remaining portion including the transverse center portion of the belt support, provided that the Te indicates a thickness at both right and left end portions, each having a width (We) of 2-25% of a total width (W) of the belt support when viewed from both right and left side edges thereof.