To provide a fibrous synthetic adsorbent excellent in adsorptivity and strength, having no danger of falling-off and excellent in processability.

A fiber composed of polyethylene or polypropylene is irradiated with ionizing radiation and a vinyl aromatic compd. such as styrene, chloromethylstyrene or divinylbenzene is graft-polymerized with this fiber and, if necessary, a chloromethyl group is further introduced into the fiber before the fiber is made porous by Friedel-Crafts reaction. As ionizing radiation, for example, α-rays, β-rays, γ-rays, electron beam or ultraviolet rays are used but γ-rays or electron beam is especially suitable.