An optical fiber, which is less likely to increase its transmission loss even when it is exposed to a high-humidity environment or immersed in water, is provided. The optical fiber comprises a glass fiber and at least two coating layers (a soft layer and a hard layer) coated at the circumference of the glass fiber, wherein the limit-adhesion strength between the glass fiber and the coating layer under a hot and humid environment is 0.5N/10 mm or more. Preferably, the glass-transition temperature of the hard layer is less than 90° C.