To realize upgrade recycling of commodity-type polyolefin resins, acid modification with maleic anhydride (MAH) and organic peroxide was applied to polypropylene (PP)-rich commercial recovered resins provided by different manufacturers. Modification conditions were established in advance to achieve high MAH grafting level as well as thermal fluidity using a laboratory scale batch kneader. Mass production prototyping with a view of productivity of 100 kg/h was carried out using an anisotropic twin-screw extruder. Transmission electron microscopy demonstrated that different domain structures of the original recovered resins were homogenized by the acid modification process. To evaluate the performance of the obtained acid-modified recovered resins as a compatibilizer, wood/plastic composite (WPC) compounds were prepared by kneading together with wood flour and PP and examined for their mechanical properties. As a result, the bending and impact strengths improved 1.5 times and the tensile strength increased 1.3 times in comparison with those of the composites containing no compatibilizer. We also conducted in 1-t/day mass production prototyping of the WPC compounds using an existing commercial production facility. By estimating the manufacturing costs of the acid-modified recovered resins, the profitability of the production was confirmed.