Measurements of the electrorheological (ER) effect, a rheological change induced by an application of an electric field, are made on the ER suspensions composed of potato starch powder and silicone oil. When an electric field of a few kV mm(-1) is applied, there appears an increase in the viscosity, which is characterized by a rapid increase of the viscosity, followed by a slow increase having a relaxation time longer than a few minutes, As a possible mechanism, the chain and the columnar structure formations are suggested to be associated with the rapid and the slow components, respectively. It is also found that there appears a memory effect; if the electric field is turned off for a short period, the shear stress slowly induced by the electric field recovers its value instantaneously.