Provided is a fiber treatment agent which exhibits excellent adhesion to an application site and is capable of improving, protecting, or repairing functions and characteristics of an application site and a method for using the same.According to the present invention, there is provided a fiber treatment agent, in which: the fiber treatment agent is prepared in a one-component formulation containing an effective amount of a reducing agent and an effective amount of a converted protein in which some or all of disulfide bonds (-S-S-) in a protein are converted by at least one of a thiol compound selected from the group consisting of cysteamine, cysteine, and thioglycolic acid or sulfurous acid, or prepared in a two-component formulation including a first agent containing the converted protein and a second agent containing the reducing agent, in which a total content of the converted proteins and a total content of the reducing agents in the first agent and the second agent satisfy the respective effective amounts； and the fiber treatment agent is used for improving strength of a fiber or improving irregularity on a hair end caused by a flipped-up or wavy fiber.
