PURPOSE:To improve homogeneous miscibility with synthetic resins and to enhance an effect of improving the physical properties of synthetic resins, by mixing gelatin or glue, swollen with a solvent with a plasticizer with stirring to homogeneously disperse them in each other to form a jelly-like compsn. CONSTITUTION:The filler compsn. for synthetic resins is obtd. by mixing gelatin or glue swollen with a solvent (e.g., glycerol) with a plasticizer with stirring to homogeneously disperse them in each other to form jelly. When hydrated glue or gelatin is used as the swollen glue or gelatin, the ratio of the glue or gelatin to the plasticizer is pref. 1:1-1:5 by weight on a dry basis. Technical grade gelatin obtd. as a by-product of hide is preferred as the swollen glue or gelatin.