A process for producing a styrene-base resin having improved moldability and a good hue by continuously adding a specific higher fatty acid amide in a dissolved or molten state, and/or a mixture of a specific higher fatty acid amide and a specific metallic soap in a molten state, to a styrene-base resin in the course of its production by continuous bulk or solution polymerization. The above process is particularly effective for rubber-modified styrene-acrylonitrile resins.