A resin composition including from 30 to 99 wt % of a cellulose-based resin represented by the following formula (1) and from 70 to 1 wt % of a fumaric acid ester polymer containing 30 mol % or more of a fumaric acid diester residue unit represented by the following formula (2); an optical compensation film using the same; and a production method of the optical compensation film. (wherein R1 to R3 represent a substituent having a carbon number of 1 to 12, and R4 and R5 represent an alkyl group having a carbon number of 1 to 12).