The present invention provides a method for manufacturing the optical film, when the solution casting film-manufacturing method is adopted for manufacturing the optical film the primary dope which takes the cellulose ester resin as the main component is in-line added with the diluting solution with lower concentration of the solid component for manufacturing the diluted casing used dope, and the optical hysteric axis of the cellulose esters resin film which is manufactured by the casing used dope is vertical (average orientation angle of the hysteric axis being within 90 DEG +-1.5 DEG) or parallel (average orientation angle of the hysteric axis being within 0 DEG +-1.5 DEG) to the conveying direction of the film; and the invention is characterized in that the viscosity variation or density variation of casting dope after the dilution is within the range of 0.01 to 1 26609256n terms of relative standard deviation.