A process for producing a plastic lens comprises, using a mold constituted with a pair of mold members facing each other and a plastic gasket which fixes the mold members to form a cavity and has an inlet for injecting a composition of a material monomer into the cavity, injecting the composition of a material monomer into the cavity through the inlet of the plastic gasket, sealing the inlet of the plastic gasket and, thereafter, conducting polymerization, wherein the inlet of the plastic gasket is sealed by the ultrasonic welding. No striae is formed in the produced lens, and the operations of the production can be performed efficiently at a high speed.