Abstract: A process for producing a fiber assembly or agglomerate requiring micropores, such as for a battery separator or any of various filters, which is performed by electrostatic spinning and provides high productivity and ease of maintenance, is provided. The process for producing a microfiber assembly or agglomerate by electrostatic spinning includes continuously forming bubbles on a polymer solution or a polymer melt and applying high voltage to the formed bubbles. The bubbles can be formed by passing compressed air through porous material of one or a combination of two or more of plastic, ceramic and metal materials, or capillaries.