The belt transfer system is one of the image transfer processes used in electrophotography. In this process, paper clings to the electro-resistive belt, and is stably fed through the processor. However, electrical discharge may occur between the paper and the belt when the paper is separated from the belt under particular conditions. This discharge is named “separating discharge”. Since this discharge causes image degradation, it is important to make clear the separating discharge phenomenon. A model experiment is carried out in order to construct a simulating model of separating discharge and image degradation. A model, which generates separating discharge patterns on the backside of paper, is proposed and numerical simulation based on this model is carried out. This model shows the strong correlation between the sheet potential before separation (Vs) and the discharge. When Vs is low, local strong discharge occurs intermittently. On the other hand, the separating discharge becomes continuous and weak when Vs is high. Toner-movement due to discharge is also numerically simulated using discrete element method, and the result shows that the image degradation occurs at low Vs, whereas it does not occur at high Vs.