The present application relates to a membrane separation device. According to the separation device of the present application, components to be separated using a separation membrane having a small area size can be separated with high selectivity and consequently processing efficiency and economical efficiency can be superbly improved; and according to a method for producing an expanded polystyrene which includes the membrane separation device, components to be separated using a separation membrane having a small area size, in particular, a volatile organic compound (VOC), can be separated with high selectivity and consequently processing efficiency and economical efficiency can be superbly improved, and also, by separating and recovering VOC, an effect in preventing environmental pollution caused by global warming is exhibited.