A mixture in the form of slurry obtained from mixing a soybean protein, sugar and water is placed between a pair of electrodes. Then, an electroconductive heating process is applied to the mixture under an applied voltage between the electrodes for a predetermined time period, thereby obtaining the sheet-like biodegradable molded article. In this method, the biodegradable molded articles have a homogeneous structure and can be prepared in a short period of time without increasing the cost of equipment of creating unfavorable working conditions.