To realize the reuse of a waste material as an ingredient of a biodegradable plastic having good strength in molding by using rice bran as an ingredient of a biodegradable plastic.

Rice barn in an amount of at most 70% based on the basic ingredients of a biodegradable plastic is kneaded together with 13-14% water in a double- or triple-screw rotary kneader (kneading machine) at about 100°C for 30-40min to convert β-starch into α-form, while keeping a water retention of 9-9.5% finally. A plasticizer is mixed therewith depending on the purpose and specification of a product. The kneaded material, mixed with a plasticizer, is then processed through, e.g. an injection molding machine to give a product.