Abstract: There is provided a polyester modifier composition for a cellulose ester resin, including: a polyester resin produced by a reaction between a diol and 1,2-dicarboxycyclohexane. The modifier composition has a number-average molecular weight (Mn) in the range of 350 to 2,000 as measured by gel permeation chromatography (GPC). The polyester resin has a molecular weight of less than 350 constitutes 5% or less by mass of the modifier composition.

Abstract: To provide a modifier for cellulose ester resins that allows a decrease in retardation value (Rth) in the thickness direction of a film, that can impart resistance to moisture permeation, and that can provide a film whose Rth value tends not to vary in response to variation in humidity; a cellulose ester optical film including the modifier; and a polarizing-plate protective film including the modifier. Provided are a modifier that includes a polyester resin (A) having a structure represented by a general formula (1) or a general formula (2) below; a cellulose ester optical film including the modifier and a cellulose ester resin; and a polarizing-plate protective film obtained by casting a resin solution onto a metal support, the resin solution being prepared by dissolving in an organic solvent the modifier and a cellulose ester resin and by subsequently drying the resin solution through evaporation of the organic solvent.