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.