The static yield stress of an electro-rheological fluid (starch/ gas-oil and mesophase-carbon/gas-oil, abbreviated as ERF), congealed by an electric field, has been examined by means of a tensile tester with parallel electrodes. Static yield stress showed different behavior from dynamic yield stress reported by many investigators : thus static yield stress is proportional to applied field strength, but not to the square of it, as is dynamic yield stress. The adhesive force between the particles was not Coulomb’s force but Maxwell’s force. The theory of McLean and Ikazaki et al. using the Johnsen-Rahbeck effect on the adhesive force of the dust pile on an electrode of the electro-static precipitator was applied and the deduced equation actually explained the phenomena. It was clarified experimentally and theoretically that the physical properties of the ERF particles had no effect on the adhesive force of the pearl chains of a congealed ERF; on the contrary, the volumetric concentration did affect it.