A process is disclosed wherein ricinoleic acid from petroleum alternative vegetable castor oil that has a hydroxyl group at the 12-position or a derivative thereof (an ester or a hydrogenated compound thereof) is polymerized in the presence of a synthetic zeolite and an immobilized lipase at around normal temperature without using any harmful polymerization catalysts or organic solvents which can cause environmental pollution whereby a polyester useful in the industry that has a weight average molecular weight of 20,000 or more is obtained. This high-molecular weight polyester is crosslinked to give a crosslinked elastomer that is comparable to synthetic rubbers.