A polymer is inserted into a cylinder (1) and heated, and a pressure is then applied to the polymer so as to extrude the same in the shape of a fiber from a nozzle (3). In the meantime, a diffusible material is injected from a nozzle (5) into the central portion of a molten liquid of this polymer through a thin-tube cylinder (4), and it is diffused gradually from the central portion of the molten polymer to the circumferential portion thereof as it flows toward a discharge port of the nozzle (3). The material extruded from the nozzle (3) is air-cooled and taken up around a drum (7) via a roller (6). The fiber-like molded product provides a distribution of refractive index varying continuously from the center thereof toward the circumferential portion thereof, and serves as a convergent or divergent plastic light transmitter. Conversely, even when the molten liquid of the polymer is extruded from the nozzle (5), a light transmitter having a region in which a refractive index varies continuously can be obtained. A proper number of the cylinders and various kinds of materials to be supplied to the cylinders can be selected.