To provide a lactic acid polymer composition excellent in transparency, heat resistance and productivity, a molded article, a sheet and a multilayer sheet each of which comprises the composition, a thermoformed article obtained by secondary-forming the sheet or the multilayer sheet, and a process for producing the thermoformed article with excellent productivity. The lactic acid polymer composition of the invention comprises 100 parts by weight of a lactic acid polymer (A), 0.1 to 3 parts by weight of an organic crystal nucleating agent (B) comprising an aliphatic carboxylic acid amide having an amide bond, and 0.1 to 7 parts by weight of a crystallization accelerator (C). The thermoformed article of the invention is obtained with excellent production efficiency by thermoforming the sheet comprising the composition of the invention at a temperature of 60 to 130° C.