PURPOSE: To efficiently carry out graft polymerization with a vinyl based monomer and/or oligomer solution without deactivating a produced active group by irradiating a polyester fiber with electron beam in an oxygen atmosphere having a specific concentration or below.

CONSTITUTION: A polyester fiber is irradiated with electron beam in a low oxygen concentration of ≤500ppm, preferably ≤100ppm to extremely efficiently carry out graft polymerization with a vinyl based monomer and/or polymerizable monomer of oligomer kept in an activated state without deactivating the resultant active group. Thereby graft polymerization capable of exhibiting excellent performances such as permanent hydrophilic property, flame retardance and antimicrobial agent can efficiently be carried out.