A cellulosic ester resin modifier for producing a cellulosic ester film, which facilitates obtaining a film that remains transparent even when exposed to high-humidity and high-temperature environments, and with which it is possible to obtain a film having excellent water vapor permeability resistance, and dimensional stability against heat. The present invention also provides a cellulosic ester resin composition using the modifier, a method for producing a polarizing-plate protective film, and a liquid crystal display device, and provides a cellulosic ester resin modifier including a polyester polyol (A) having a phthalic acid residue and an aliphatic glycol residue having 2 to 3 carbon atoms in a main chain skeleton and a diester (B) having an aliphatic glycol having 2 to 3 carbon atoms and an aromatic monocarboxylic acid, in which a mass ratio of [(A)/(B)] is in a range of 90/10 to 60/40.