To allow modifying synthetic fibers or a fiber product, by which the dyeability or the like of fibers or a fiber product containing synthetic fibers can safely be modified in a simple operation at room temperature to save energy.

This method for modifying the synthetic fibers or fiber product containing the synthetic fibers includes irradiating the fibers or fiber product containing the synthetic fibers with light in the presence of a compound producing a radical by the irradiation with light and a compound having a carbon-carbon double bond in which an electron-attracting group is directly bound to carbon atoms forming the double bond, or the method for modifying the fibers or fiber product containing the synthetic fibers includes irradiating the fibers or fiber product containing the synthetic fibers with light in the presence of a compound producing a radical by the irradiation with light and then being treated with a compound having a carbon-carbon double bond in which an electron-attracting group is directly bound to carbon atoms forming the double bond. The compound producing the radical by the irradiation with light is preferably a peroxide. The method also includes an embodiment that the fibers or fiber product is treated with a reducing agent after the irradiation with light.