A method of molding a resin molded product, including: a first molding step of injecting a first resin molding material into a first molding die to form a first resin product, wherein a fixing protrusion is formed on at least one of an inner and an outer wall part of the first resin product, and has a shape in which a width increases in a direction extending from the gap; and, a second molding step of positioning the first resin product in a second molding die, and injecting a second resin molding material at least into the gap of the first resin product to form a second resin product, wherein the fixing protrusion is embedded into a fixing recess disposed on the second molding die, thereby limiting movement of the fixing protrusion in a direction towards the gap.