To provide a treatment method for hair fibers which, while limiting damage to the hair, changes the properties of hair fibers, controls the volume of the hair, enhances cosmetic advantages, especially in terms of softness, gloss, and easiness in combing imparted to the hair, and simultaneously keeps the color tone of the colored hair better.

The treatment method for hair fibers which does not perform hair fixing comprises the following steps: an application step wherein a reducing composition which does not contain a ceramide and contains at least one reductant selected from thiols and at least one cosmetic activator selected from polymerizable activators is applied to hair fibers; and a step for elevating the temperature of the hair fibers with a heating iron at 60°C or higher before or after an optional step for rinsing hair fibers.