To provide a modifier for cellulose ester resin by which the thickness-direction retardation value (Rth) of a cellulose ester resin-containing film can be reduced to a practically adequate level and excellent moisture permeation resistance can be imparted to the film, and which is excellent in bleeding resistance and less liable to volatilize even under severe conditions of high temperature and high humidity, and to provide a cellulose ester optical film and a polarizing plate protective film using the modifier.

The modifier for cellulose ester resin comprises an aliphatic polyester (A) having a number average molecular weight of 1,000-3,000 obtained by bringing a 2-4C glycol (a), a 2-6C aliphatic dicarboxylic acid (b) and a 4-9C monoalcohol and/or monocarboxylic acid (c) into a reaction.