To provide a silk-based fiber product equipped with an improved lightfast property, effectively preventing yellowing caused by light without damaging the excellent property of the silk fiber, and enduring its use without reducing its effect for a long period of time.

This lightfast yarn is obtained by spinning with combining an artificial fiber containing a far infrared light-irradiating material with the silk fiber, preferably so as to surround the artificial fiber with the silk fiber, and the cloth produced by using the lightfast yarn and the fiber product secondarily processed by using such yarn and cloth also show the same effect.