The present invention relates to an optical fiber, wherein PMD will not significantly increase even when an external factor, such as a lateral pressure, a bending and the like, is applied to the optical fiber. The optical fiber is imparted with a twist that is an elastic torsion with stress. In the condition that a twist rate, being the number of rotations per unit length by the twist, is represented by f(z) as a function of a position z in a longitudinal direction of the optical fiber, which has a predetermined twist cycle and is defined such that, in one twist cycle, a twist rotation angle in one direction and a twist rotation angle in the opposite direction are equal to each other, when a proportionality coefficient representing circular birefringence per twist rate is denoted by g, an angular frequency is denoted by ω, and external linear birefringence due to a bending and a lateral pressure is denoted by βe, then an average twist cycle L and an average twist rate γav in the optical fiber satisfy predetermined relational expressions.