Four sets of shower heads 100, 200, 300, 400 are located in a surface modifying chamber of a surface modifying apparatus, and each shower head 100, 200, 300, 400 has surface modifier piping including a plurality of spray nozzles. The nozzles are arranged such that showers of an aqueous ozone solution injected from every adjacent two nozzles will contact the resin molded product and partly overlap one another. The phases in position of the nozzles can be offset a half-pitch or staggered with the staggered arrangement of the nozzles serving to compensate for any insufficient oxidation reaction to achieve uniform surface modification over the entire surface of the resin molded product 2. Further, the time required for the surface modification treatment can be reduced.