A polymer clad optical fiber is provided in which, if the diameter of an
inner core is taken as a.sub.1, and the diameter of an outer core is
taken as a.sub.2, and if a ratio X (which=a.sub.2.sup.2/a.sub.1.sup.2)
between a cross-sectional area of the inner core and a cross-sectional
area of the outer core is within a range of 1.8.ltoreq.X.ltoreq.2.2, and
if a relative refractive index difference between the inner core and the
outer core is taken as .DELTA..sub.1, and if a relative refractive index
difference between the outer core and a cladding is taken as
.DELTA..sub.2, then for a parameter Y which is defined as
Y=.DELTA..sub.1/.DELTA..sub.2, when a high temperature is taken as Ymax
and a low temperature is taken as Ymin, a relationship is established in
which, when X is within a range of 1.8.ltoreq.X.ltoreq.2.0, then
0.25.ltoreq.Ymin.ltoreq.0.84X-0.68, and Ymax is
0.25.ltoreq.Ymax.ltoreq.0.84X-0.68, and, when X is within a range of
2.0.ltoreq.X.ltoreq.2.2, Ymin is
0.48X-0.71.ltoreq.Ymin.ltoreq.-2/9X+13/9, and Ymax is
0.48X-0.71.ltoreq.Ymax.ltoreq.-2/9X+13/9.