To obtain a functional polyester-based fiber that uses no binder and has a slight change of a feeling and durability by introducing an amino group into a fiber, reacting the amino group with a functional agent and bonding/securing the functional agent to the polyester-based fiber in securing a functional agent to a polyester-based fiber.

A diamine compound is added to and reacted with a polyester-based fiber to introduce an amino group into the polyester-based fiber, thereafter or simultaneously a functional agent to be reacted with the amino group is added and bonded to the polyester-based fiber to bond/secure the functional agent through the amino group to the fiber. Consequently, various functional polyester-based fibers having a slight change of a feeling and durability are readily obtained.