PURPOSE: To greatly improve the storage stability of a film of a lower fatty acid ester of cellulose contg. a phosphoric ester plasticizer by incorporating specified amts. of specific compds. into the film.

CONSTITUTION: 0.1-10wt.% aliph. epoxy compd. (e.g. a compd. of formula I) and 0.01-5 wt.% org. base of formula II or III (wherein R21 to R27 are each alkyl, alkenyl, alkynyl, or aralkyl provided two of R21 to R23 may combine to each other to form a ring and the same applies to R24 to R27 are incorporated into a film of a lower fatty acid ester of cellulose (e. g. cellulose triacetate) contg. a phosphoric ester plasticizer.