In recent years, with an increase of sensitivity to global environment, a polylactic acid-based resin receives attention as a material that is a substitute for general resins produced from the conventional petroleum resources. The polylactic acid-based resin is produced from a plant such as corn as a starting material and is a thermoplastic resin that is regarded as being of a low environmental load type from the standpoint of carbon neutral. Such a polylactic acid-based resin is expected to be used as a plant-derived general resin for foams. Thus studies are being made on foams made of polylactic acid-based resins as a raw material. Among such foams, polylactic acid-based resin expanded beads molded articles can be obtained by in-mold molding in any desired shape without restriction, similar to conventional polystyrene resin expanded beads molded articles. Such molded articles are particularly promising in that they are likely to allow easy design of properties according to the aimed lightness in weight, cushioning property and heat insulating property. In this circumstance, inventions of Patent Documents 1 to 6 have been hitherto made.
Patent Documents 1 to 5 describe that a gas impregnation pre-expansion method was adopted in an attempt to obtain polylactic acid-based expanded beads.