A preparation process of heat-resistant molded articles of a lactic acid-based polymer comprising mixing the lactic acid-based polymer and poly-.epsilon.-caprolactone so as to obtain a L-lactic acid ratio of 75% by weight or more, adding to the polymer mixture a crystalline inorganic powder containing 50% by weight or more of SiO.sub.2, melt-kneading the resultant composition, filling the same into a mold which is set at 85.degree..about.125.degree. C. and forming a shape while promoting crystallization. The molded articles have excellent heat resistance and impact strength, are suitably used for food trays and beverage cups, and can be degraded, in the case of waste, into nontoxic water and carbon dioxide like natural products within a relatively short period in the natural environment.