An optical fiber for preserving the plane of polarization including a cladding and a stress imparting part disposed in the cladding, the stress imparting part comprising a base material including B2O3. The average concentration of B2O3 in the base material of the stress imparting part is in a range between about 17 and 21 wt. %, and the maximum concentration of B2O3 in the base material is in a range between about 17 and 22 wt. %. The optical fiber has excellent polarization plane preserving properties, and no concave portion is generated in its stress imparting part even under wet heat conditions.