PURPOSE:To provide the titled optical fiber which exhibits the absolute single polarizing plane maintaining characteristic even if the fiber is not bent by using a support having the refractive index equal to the refractive index of a clad and determining the refractive index distribution of an elliptical jacket in such a manner that the refractive index is equal in the minor axis direction and has the low refractive index in the major axis direction. CONSTITUTION:The elliptical jacket 3 consists of SiO2 glass added with P2O5 and B2O3 and has a large coefft. of thermal expansion. The amt. of the B2O3 to be added is large in the major axis direction (X-axis direction) thereof and the jacket has the refractive index n1 lower than the refractive index n0 of the clad 2 in said direction. The jacket has the refractive index n0 equal to the refractive index of the clad 2 in the minor axis (Y-axis direction). The fiber exhibiting the absolute single polarizing plane maintaining characteristic to propagate only the polarization mode in the single direction is thus obtd. even if the fiber is not bent.