To provide a manufacturing method of an optical film having less gelatinous foreign materials causing bright spots, even using a cellulose ester-containing dope in a long-term continuous production.

The manufacturing method of an optical film includes the steps of: filtering a dope including at least cellulose ester through a filtration medium; backwashing the filtration medium with a portion of the filtrate by passing the dope through a portion of the filtration medium with a backwashing unit in the direction reverse to the direction in the filtering step; and flow casting the dope on a support after passing through the filtering step. In the backwashing step gelatinous foreign materials contained in the dope is backwashed under conditions allowing for easier deformation compared to the conditions for filtering the dope in the filtering step.