A device for producing a polymerizable synthetic material optical lens, including a closure member (12) that is adapted to grip two molding shells (14A, 14B) circumferentially to define a molding cavity (15) in conjunction with the shells (14A, 14B), the closure member (12) including a synthetic material wall (123) adapted to come into contact with the molding shells (14A, 14B) and the closure member (12) incorporating a molding material introduction passage (125) discharging into the molding cavity (15) through a casting opening (126); characterized in that the closure member (12) includes a seal (40) that forms the contact wall (123) and the introduction passage (125), which joint (40) is formed of an elastomer material and of a rigid insert (43) at least part of which has the elastomer material molded over it, the introduction passage (125) being inside the insert (43).