To provide a resin composition for modifying a cellulose ester, capable of reducing a volatile component at the production process of a cellulose ester film while maintaining high retardation and low moisture permeability, and to provide the cellulose ester film containing the resin composition.

The resin composition for modifying the cellulose ester contains an epoxy ester resin of a product obtained by reacting the following components (A) with (B), or the components (A) and (B) with the components (C) and/or (D), and having 3 mg-KOH/g acid value, 15,000 epoxy value and 1,500 number average molecular weight: (A) an epoxy compound having two or more aromatic rings; (B) an aromatic monobasic acid or a derivative thereof; (C) an aromatic dibasic acid or a derivative thereof; and (D) a glycol. The cellulose ester film contains the resin composition.