In a resin molded lens 20 of an interior illumination lamp which includes inserting openings inserting seesaw type switch knobs, gates G and G1 for a resin molding are formed so as to be installed at inner side surfaces of the inserting openings when the resin molding is performed by molds D1 and D2, and protrusions 20T having heights higher than heights of gate traces G2 generated by the gates G and G1 are formed in the vicinity of gate traces G2.