To provide a membrane separation method capable of attaining reduced adsorption of a membrane-fouling substance contained in treatment water onto the surface of a separation membrane during membrane separation of the treatment water, to thereby lead retarded deterioration in membrane separation performance, and a membrane separation apparatus for performing the method. In the membrane separation method, to treatment water, a particulate cationic polymer which swells in water but does not substantially dissolve therein is added, and the treatment water is subjected to membrane separation.