PURPOSE:To obtain a synthetic resin optical fiber which increases or decreases in refractivity in proportion to the square of the distance from a center axis advantageously by bringing atomized monomers forming polymer which differ in refractivity into contact with a transparent gel body. CONSTITUTION:A monomer Ma forming a network polymer with refractivity Na is formed into a cylindrical or fibrous transparent gel body by partial polymerization. Then, a monomer Mb forming a polymer Pb having refractivity Nb different from the refractivity Na is atomized into particulates in a physical method such as ultrasonic or spray nozzle injection at temperature Ta, and the particulates of the monomer Mb are brought into contact with said transparent gel body at specific temperature T2. The atomized particulates of the monomer Mb arrive at the surface of the gel body and are diffused into the gel to be polymerized. The vapor of the monomer Mb vaporized from the particulates is also diffused insides. In a nitrogen atmosphere containing no particulates of the monomer Mb, a heat treatment is carried out at specific temperature T3 for a specific time t3 after a specific time t2, thus completing the polymerization.
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