Abstract: A large-sized hollow fiber membrane separation apparatus suitable for treating a large amount of fluid without any defect, such as pressure loss of fluid, a concentration polarization phenomenon and the like, which comprises a cylindrical pressure container divided into two longitudinally aligning compartments by a dividing member at the substantial longitudinal center portion thereof; two columnar hollow fiber assemblies longitudinally aligned and placed in the compartments, respectively; said dividing member having functions to seal the compartments in a fluid-sealing condition, to feed fluid to be treated and to discharge fluid which does not permeate through the hollow fiber membrane during membrane separation in the compartments and being directly connected to external pipings through opening provided to the side wall of the container at the substantial longitudinal center portion thereof.