To provide a modifier which is easily decomposed by microorganisms, is excellent in terms of global environment and biological environment, and is used for producing a natural rubber-base polymer with enhanced strengths and provide a composition of the same.

This modifier is prepared by the graft polymerization of (B) 5-99 wt.% monomer such as a carboxylated monomer (e.g. methacrylic acid), a hydroxylated monomer (e.g. 2-hydroxyethyl acrylate), or a glycidylated monomer (e.g. glycidyl acrylate) and (C) 0-94 wt.% other monomer (e.g. styrene) in the presence of (A) 1-95 wt.% rubbery polymer comprising natural rubber and/or an epoxidized natural rubber, the graft efficiency being 30 % or higher. A biodegradable polymer composition is provided which contains the modifier (in an amount of about 0.1-70 wt.%) and a natural-derived polymer (e.g. chitosan), a microorganism-synthesized polymer (e.g. polyhydroxybutyrate), or a chemically synthesized biodegradable polymer (e.g. polylactic acid).