A method of manufacturing a plastic lens by mixing plural polymerizable components, and immediately after mixing, casting a mixture into a casting mold and conducting polymerization to obtain a molded article. A method of manufacturing a plastic lens by casting a plastic lens starting material liquid flowing out of a casting jig opening into a casting mold and curing the above starting material liquid to form a molded article. A gasket for molding plastic lens comprised of a cylindrical member comprising openings for fitting by insertion of two molds. A casting mold for molding plastic lens, wherein the above molds are detachably held in the above openings of the above gasket so that molding surfaces of the two molds are positioned opposite at a prescribed interval to form a cavity corresponding to a lens shape within the above cylindrical member. A casting jig for plastic lens starting material liquid equipped with a nozzle for casting a plastic lens starting material liquid into the casting mold. A method of manufacturing a plastic lens comprising casting of a plastic lens starting material liquid into a casting mold held by a holding member and curing the above starting material liquid to form a molded article. A holding member for plastic lens casting mold for holding a casting mold for molding a plastic lens.