A process of injection molding a foamable plastic composition which comprises filling completely a molten foamable plastic composition into a cavity, while reducing a volume of the cavity in the middle of injection or immediately after injection; cooling the composition to the state wherein a solidified layer in contact with a mold surface mingles with a molten inside layer; enlarging the volume of the cavity to that of the desired molded article; and after further cooling, removing the molded plastic foam article. This molded article comprises a layer having a non- or low-foamed, dense structure on the surface and a high-foamed layer at the inside, and has a light weight, good appearance and stiffness.