The ribs of the injection molded article are a necessary shape in order to maintain the rigidity without increasing the material cost. The ribs on the outside of the molded article in the form of a housing are located on the side of the cavity inside in the mold. This paper proposes slotting by machining side-ribs. The slotting was performed through CNC control along the edge profile of the rib width. The machining time and the roughness of the machined surface were compared to verify the machining efficiency and the practicality of the machining method as a side-rib machining method.