The purpose of the present invention is to provide ligand-immobilized sea-island composite fiber wherein both generation of microparticles caused by detachment of sea components from island components and generation of microparticles because of the destruction of fragile sea components are suppressed. The present invention is a sea-island composite fiber formed from sea components and island components, wherein: the value (L/S) of the average total length (L) of the perimeter of all island components in a cross-sectional surface perpendicular to the axis of the fiber divided by the average cross-sectional surface area (S) for the cross-sectional surface is 1.0 – 50.0 µm
