This manufacturing method for a resin molded body includes: a step in which an intermediate molded body made of a resin composition is prepared, the intermediate molded body having a rough surface with a maximum peak height (Rp) of 10-5000 µm measured according to JIS B 0601 or a maximum valley depth (Rv) of 10-5000 µm measured according to JIS B 0601; and a step in which a thin film-like molded body is fused to the rough surface of the intermediate molded body by irradiation with a laser, the thin film-like molded body being made of a resin composition containing reinforcing fibers arranged in one direction.