A diethylamino group as an anion-exchange group was introduced to the poly (glycidyl methacrylate) chain grafted onto a porous hollow-fiber membrane. Subsequently, aminoacylase was adsorbed in multilayers by the graft chains. The enzyme was crosslinked by a reaction with glutaraldehyde to prevent the leakage of the enzyme from the graft chain. Then, an acetyl-DL-methionine solution (pH 8.0) was permeated outward through the aminoacylase-immobilized porous hollow fiber at a permeation rate of 150 ml/h. At a lower feed concentration such as 10 mM, 100% conversion was attained. As the feed concentration was increased, the overall reaction was reaction-controlled. The productivity of L-methionine was maintained at 1, 500μmol/h/ml-fiber up to space velocity of 150 h^-1, where space velocity is defined as effluent volume divided by membrane volume including the lumen.