A resin composition includes polylactic acid resin, a plastic resin superior in heat resistance to the polylactic acid resin, and a nucleating agent that promotes crystallization of the polylactic acid resin, with the nucleating agent being a soluble azo lake pigment. The plastic resin superior in heat resistance to the polylactic acid resin is one which has a flexural modulus of elasticity no lower than 1000 MPa at 80.degree. C.