Disclosed is a process for producing a resin composition, which comprises: a step of melt-kneading a raw material (A) which comprises a polylactic acid resin, a crystal nucleating agent, and a plasticizer comprising an ester compound formed from a polycarboxylic acid having a C1-7 hydrocarbon group and a monoalcohol having a C1-7 hydrocarbon group, thereby preparing a polylacic-acid-containing melt-kneaded material having a relative crystallinity degree of 70% or more (step A); and a step of further melt-kneading the polylactic-acid-containing melt-kneaded material produced in step (A) together with a raw material (B) comprising a polypropylene resin and a compatibilizing agent (step B). The resin composition produced by the process can be used suitably in various industrial applications such as commodities, parts for household electrical appliances and automotive parts.