the present invention relates to an apparatus for monitoring the progress of membrane fouling that occurs on pores as well as on the surface of a membrane by means of variations of zeta potential of a hollow - fiber membrane according to time passage of filtration of a suspension , wherein colloid particles , biopolymers and other inorganic particles are dispersed , and the method thereof . moreover , the present invention also relates to a method to identify the effect of concentration polarization layer and cake layer which can vary according to the axial position of a hollow - fiber and the developing progress of a membrane fouling by measuring the position - dependent zeta potential of the hollow - fiber membrane .