A polymer clad optical fiber is provided in which, if the diameter of an inner core is taken as a1, and the diameter of an outer core is taken as a2, and if a ratio X (which=a22/a12) 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≦X≦2.2, and if a relative refractive index difference between the inner core and the outer core is taken as Δ1, and if a relative refractive index difference between the outer core and a cladding is taken as Δ2, then for a parameter Y which is defined as Y=Δ1/Δ2, 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≦X≦2.0, then 0.25≦Ymin≦0.84X−0.68, and Ymax is 0.25≦Ymax≦0.84X−0.68, and, when X is within a range of 2.0≦X≦2.2, Ymin is 0.48X−0.71≦Ymin≦−2/9X+13/9, and Ymax is 0.48X−0.71≦Ymax≦−2/9X+13/9.