The filtration behavior of calcite/water su spension containing various amount of Sodium hexameta-phoshate (NaPOM₃)₆, were observed and the specific filter cake resistance was analized in a view point of interfacial electrochemical phenomenon.<br>In the filtration experiment, the relation between filtrate volume and filtration time, and also the porosity of filter cake were measured, using a vacuum type filtration apparatus.<br>Two kinds of average specific filter cake resistances, denoted as α and α' were determined from the filtration data. Here, the α is a resistance calculated by the filtration equation derived from Hagen-Poiseuialle's law and α' is one calculated by Kozeny-Carman's equation.<br>The resistance α takes larger value than α', and a dependence on zeta-potential of the electrical double layer which is elevated to be dispersive by increasing the addition amount of (NaPO₃)₆ is quite different between them, as like increasing α, against decreasing α'.<br>The value of relative viscosity η'/η equivalent to α'/α increases gradually accompanied with zeta-potential until maximum value of about 8, where η and η' are viscosity of water in bulk and in filter cake respectively. The η'/η is also changed “thixotropically” by the flowing velocity of the fluid through the micro porous filter cake.<br>For these results, it is discussed that the electrical double layer and oriented water molecules surrounding the suspended particle will originate the large porosity due to keep far the particle-particle distance, and high viscous water film at the interface.