A cylindrical outer clad section is formed by melt-extrusion molding of polyvinylidene fluoride (PVDF) having a molecular structure in which the number of defect bonds of successive CF.sub.2 units and successive CH.sub.2 units constitutes not less than 4% with respect to a total number of bonds of CF.sub.2 units and CH.sub.2 units. Next, an inner clad section forming material is poured into a hollow portion of the outer clad section and polymerized to form a cylindrical inner clad section. A core section forming material is filled in a hollow portion of the inner clad section and polymerized to form a core section. Thus, a preform is fabricated. The preform is heat-drawn to obtain a POF. The POF has an outer clad in which small spherocrystals are formed. Average roughness Ra of an inner wall of the outer clad is less than 0.10 .mu.m.