The method for producing a resin composition of the present invention includes a step of obtaining a lignin-containing solution that contains a solubilized lignin prepared through a treatment of separating lignin in a lignin-containing solution and a solvent, and a step of adding a resin to the lignin-containing solution to thereby obtain a lignin/resin-containing solution. Also, the method for producing a resin composition of the present invention includes a step of mixing the lignin/resin-containing solution with at least one solvent selected from water and a hydrocarbon having a dipole moment of 0.25 d or less. The lignin-containing resin molded article of the present invention uses the resin composition obtained according to the production method of the present invention.