Abstract: The resin composition production method includes a step in which a cellulose fiber composition containing 5 to 45 parts by mass of water with respect to a total of 100 parts by mass of cellulose fibers (A) and an acrylic resin and/or a styrene acrylic resin (B) is kneaded with a thermoplastic resin (C), and water is removed until the water content after kneading falls to 1% or less. The resin composition production method is also characterized in that: 20 parts by mass or more and 200 parts by mass or less of the resin (B) is blended with respect to 100 parts by mass of the cellulose fibers (A), the dissolution amount of the resin (B) in 100 g of water at 25° C. is less than 1 g, and the glass transition temperature of the resin (B) is 40 to 150° C.