Filtration under constant D. C. electric field with constant hydraulic pressure, what is termed electrokinetic filtration, was discussed in this paper. Under these conditions it is considered that electrophoresis occurs in the slurry and causes less cake formation, and that electroosmosis occurs at the same time in the filter cake. Therefore, the flow rate of electrokinetic filtration is increased in comparison to filtration at the same hydraulic pressure. An equation taking account of both effects is presented. Experimental investigation of the characteristics of electrokinetic filtration is performed according to the theoretical equation. As a result, the flow rate of filtration is significantly increased for calcium carbonate slurry and white clay slurry. For instance, at the electric field of 30 volt/cm under hydraulic pressure of 163.2 G/cm², Ruth''s filtration coefficient for calcium carbonate slurry was about 9.4 times as large as that of the filtration at the same hydraulic pressure, and Ruth''s coefficient for white clay slurry was about 15 times at 12volt/cm and 163.2 G/cm². It is confirmed that the equation of electrokinetic filtration proposed in this paper is practically available.