The present invention provides a fiber processing agent used for processing a surface of a fiber, and the fiber processing agent contains a soluble eggshell membrane and a reactive organic compound having a reactive group. With the fiber processed with the fiber processing agent having the composition as described above, even when the fiber is used for a long period of time, the eggshell membrane is not removed therefrom and the properties provided by the eggshell membrane can be maintained, so that the fiber has excellent durability and can continuously provide the moisture retention, water-absorbing property, moisture-absorbing property, antistatic property, and wound-treating property.