To provide a pyridobisimidazole fiber with improved post processability while maintaining the excellent heat resistance and flame retardance of the pyridobisimidazole fiber and further without requiring the wide change of the conditions of production process, nor requiring a heat treatment for a long time at a high temperature.

In the pyridobisimidazole fiber, pyridobisimidazole crystal exists to satisfy a ratio of S2/S1 of 0.1 to 1.5, when a diffraction peak area derived from a crystal (200) surface is defined as S1 and the diffraction peak area derived from the crystal (110), (210) and (400) surfaces are defined as S2, in an equatorial directional profile in an electron diffraction chart obtained from a surface part (surface to 1 m) of the pyridobisimidazole fiber.