A method and apparatus for low-cost production of a lens for a vehicular lamp formed by integrally joining a plurality of synthetic resin lens members through vibration welding. The s apparatus includes a fixed portion and a vibrating portion, each have a lens fixing jig composed of a pair of respective holding jigs. The method includes the steps of supporting the respective lens members with one of the jigs, holding a lens member between a pair of holding jigs located close together, bringing each welding portion of the respective lens members into contact with each other by moving the vibrating portion and the fixed portion close together, and integrally welding the lens members by vibrating the vibrating portion.