In the condition that an acrylic transparent vessel is filled with a curable resin solution capable of being cured by a light, a plastic optical fiber is immersed in the curable resin solution. A laser beam is applied on the curable resin solution through the plastic optical fiber. The curable resin solution is cured gradually by the laser beam applied on the curable resin solution, so that an axial core is formed. Then, the transparent vessel is left at rest for predetermined time, or uncured part of the curable resin solution is removed from the transparent vessel and the transparent vessel is then filled with another curable resin solution. Then, ultraviolet rays are applied on the transparent vessel from the outside of the transparent vessel to cure the residual uncured part of the curable resin solution.