PURPOSE: To obtain the subject fiber having excellent hygroscopicity and high light-fastness by graft-polymerizing an unsaturated vinyl monomer containing carboxyl group to a polyester fiber and adsorbing ions such as cobalt ion to the fiber.

CONSTITUTION: The objective hygroscopic polyester fiber having excellent hygroscopicity and high light-fastness can be produced by graft-polymerizing an unsaturated vinyl monomer containing carboxyl group (e.g. acrylic acid) to a polyester fiber and adsorbing ions to the fiber by immersing the fiber in an aqueous solution containing ions of metal selected from cobalt, nickel and zinc.