Tripeptide was produced during the permeation of a gelatin solution through the pore of a collagenase-immobilized porous hollow-fiber membrane. Gelatin was obtained via hydrolysis of fish collagen. First, an epoxy-group-containing monomer was graft-polymerized onto an electron-beam-irradiated porous hollow-fiber membrane. Second, the 2-hydroxyethylamino group was introduced into the epoxy group to bind collagenase on the basis of electrostatic interaction. Third, adsorbed collagenase was cross-linked with glutaraldehyde to prevent leakage of the enzyme. Gelatin solution (10-50 g/L) was forced to permeate across the collagenase-immobilized porous hollow-fiber membrane with a density of immobilized collagenase of 52 mg/g at various residence times of the gelatin solution ranging from 0.13 to 20 min. Fourteen percent in weight of 10 g/L gelatin solution was hydrolyzed into tripeptide at a residence time of 20 min.