PURPOSE: To prevent permanent stress from occurring in a knockout process and to enable temperature control by incorporating attachments, raising several engaging pins in an optical direction to these attachments and pushing knockout pins to them.

CONSTITUTION: Melted resin poured fully into a cavity is cooled as attachments 3, 3 are incorporated, a convex lens 1 is molded as a molded piece and fitted to the retainer plate 8 on the movable side. Then, when a knockout plate is advanced toward the retainer plate 6 on the fixed side, the ends of the knockout pins 11, 11 abut against and push the engaging pins 5, 5 buried in the perforated holes 10, 10. This pushing force pushes up the plastic body 2 outward through the attachments 3, 3 to project the convex lens 1 from the retainer plate 8 on the movable side. The knockout pins 11, 11 do not touch the plastic body 2 directly and accordingly, mold traces, cracks, whitening do not occur. The knockout force acts uniformly to bring no deformation on the plastic body 2. Additionally, cooling holes 13... can be approached to the core 9 on the movable side and the temperature of the mold is controlled to be kept always at suitable conditions.