Electro-rheological Fluid (ERF) is the functional fluid that can change its viscoelastic property by applying electric field. However, ERF shows the sedimentation of the ER particles in long time use, and it caused the unstable ER effect. The Gel-structured ERF (ERG) has been developed to solve this problem. When a metallic flat plate is placed on the ERG sheet, ERG shows the high shear force due to the occurrence of the contact between the gel and the plate in response to the applying electric field (ERG effect). ERG effect can be obtained not only for metallic materials but also non-metallic materials by applying the one-sided electrode, and the ER effect is supposed to be considerably influenced by the material and geometrical properties of flat plate on the ERG sheet. In this study, the influences of the material and geometrical properties of sliding plate are experimentally analyzed from the viewpoint of the relative permittivity, surface roughness and flatness, and the design instruction for the selection of materials and their geometrical issues are proposed.